Day (d) - Unit Information & Conversion

Quick Answer

A day is 24 hours, 1,440 minutes, or 86,400 seconds—one complete rotation of Earth relative to the Sun. It's the most fundamental unit of human timekeeping, marking the natural cycle of light and darkness, sleep and wakefulness, work and rest. The word "day" comes from Old English dæg, related to Sanskrit dah (to burn), referring to daylight.

Real-world perspective: One day is a sunrise to the next sunrise, a full work-sleep cycle, the time Earth rotates 360° on its axis, one entry in a calendar, or the period between two midnights. It's the heartbeat of human civilization.

Quick Comparison Table

Time Period	Days	Hours	Minutes	Seconds
One day	1	24	1,440	86,400
One week	7	168	10,080	604,800
Fortnight	14	336	20,160	1,209,600
Lunar month	~29.5	~708	~42,480	~2,548,800
Calendar month	28-31	672-744	40,320-44,640	2,419,200-2,678,400
Quarter (90 days)	90	2,160	129,600	7,776,000
Year (365 days)	365	8,760	525,600	31,536,000
Leap year	366	8,784	527,040	31,622,400
Decade	3,652	87,648	5,258,880	315,532,800
Century	36,525	876,600	52,596,000	3,155,760,000

Definition

The day (symbol: d) is a unit of time equal to 24 hours, 1,440 minutes, or 86,400 seconds.

Official civil definition: Since 1967, one day is defined as exactly 86,400 SI seconds, where each second equals 9,192,631,770 periods of caesium-133 radiation. Therefore:

• 1 day = 86,400 × 9,192,631,770 = 793,927,920,332,800,000 caesium-133 oscillations
• This equals approximately 794 quadrillion atomic oscillations

Astronomical definitions:

1. Solar day (apparent solar day):

   • Time between two successive transits of the Sun across the local meridian (noon to noon)
   • Varies throughout year: ±16 minutes due to Earth's elliptical orbit and axial tilt
   • Mean solar day: Average of all solar days = 24 hours exactly (86,400 seconds)
   • This is the basis for civil timekeeping

2. Sidereal day:

   • Time for Earth to rotate 360° relative to distant stars
   • 23 hours, 56 minutes, 4.09 seconds (86,164.09 seconds)
   • ~4 minutes shorter than solar day
   • Used in astronomy for telescope tracking and star charts

3. Synodic day (planetary science):

   • Time for same position of sun in sky on other planets
   • Mars sol: 24 hours, 39 minutes, 35 seconds
   • Venus day: 116.75 Earth days (very slow rotation)

Why the difference?

• Earth rotates 360° in one sidereal day
• But Earth also orbits the Sun (~1° per day along orbit)
• Must rotate an additional ~1° (4 minutes) for sun to return to same position
• Result: Solar day = sidereal day + ~4 minutes
• Over one year: 365 solar days, but 366 sidereal days (one extra rotation)

History of the Day

Prehistoric Recognition (Before 3000 BCE)

The day-night cycle is the most fundamental observable pattern in nature, recognized by all human cultures and even animals:

Biological origins:

• Circadian rhythms: Internal ~24-hour biological clock evolved in response to Earth's rotation
• Found in bacteria, plants, animals, humans
• Regulated by light/dark cycle
• Predates human civilization by billions of years

Early human observation:

• Stone Age: Organized activities by sun position (hunting at dawn, gathering by day)
• Neolithic era: Agricultural cycles tied to day length (planting, harvesting)
• Megalithic monuments: Stonehenge (c. 3000 BCE) aligned with solstice sunrise
• Earliest "clocks": Shadows cast by objects (proto-sundials)

Ancient Egyptian Timekeeping (c. 3000 BCE)

Egyptians formalized day measurement:

1. Shadow clocks and sundials (c. 1500 BCE):

   • Obelisks cast shadows indicating time of day
   • Divided daylight into 12 parts (seasonal hours)
   • Used horizontal bars with markings

2. Water clocks (clepsydrae):

   • Used at night when sundials didn't work
   • Water dripped at constant rate through calibrated container
   • Divided night into 12 parts

3. Decans (star clocks):

   • 36 groups of stars rising throughout year
   • Each decan rose ~40 minutes apart
   • Used to tell time at night

Egyptian day structure:

• Day began at sunrise (variable time)
• 12 hours daylight + 12 hours darkness = 24 hours
• But "hours" varied by season (longer daytime hours in summer)

Babylonian Contributions (c. 2000 BCE)

Babylonians established key concepts:

1. Seven-day week:

   • Based on seven visible celestial bodies (Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn)
   • Each day named after a planet/god
   • This system spread globally

2. Day began at sunset:

   • Still used in Hebrew and Islamic calendars
   • Genesis 1:5: "And there was evening, and there was morning—the first day"

3. Base-60 mathematics:

   • Eventually led to 24 hours, 60 minutes, 60 seconds
   • 360° circle (from ~360 days in year)

Greek and Roman Systems (500 BCE - 400 CE)

Greek astronomers:

• Hipparchus (c. 150 BCE): Studied equation of time (variation in solar day length)
• Recognized need for "mean solar day" as average

Roman timekeeping:

• Day began at midnight (adopted by modern civil timekeeping)
• Divided into:
  • Dies (daytime): Sunrise to sunset, 12 horae (hours)
  • Nox (nighttime): Sunset to sunrise, 4 vigiliae (watches) of ~3 hours each
• Market day cycle: Nundinae (8-day week, superseded by 7-day week)

Roman calendar influence:

• Julian Calendar (45 BCE): 365.25-day year, leap years
• Day names from planets (still used): Sunday (Sun), Monday (Moon), Saturday (Saturn)

Medieval and Islamic Developments (600-1300 CE)

Islamic timekeeping:

• Day begins at sunset (following Hebrew tradition)
• Five daily prayers (salat) structured the day:
  • Fajr (dawn), Dhuhr (noon), Asr (afternoon), Maghrib (sunset), Isha (night)
• Sophisticated astronomical tables calculated prayer times
• "Islamic day" vs. "civil day" distinction in Muslim countries

Medieval Christian hours:

• Canonical hours: Structured monastic life
  • Matins (midnight), Lauds (dawn), Prime (6 AM), Terce (9 AM)
  • Sext (noon), None (3 PM), Vespers (sunset), Compline (bedtime)
• Church bells marked these hours, organizing community life

Mechanical Clocks and Equal Hours (1300s)

Transformation of daily time:

Before mechanical clocks:

• "Hours" varied by season
• Time was task-oriented ("work until sunset")
• Imprecise coordination

After mechanical clocks (1300s-1400s):

• 24 equal hours became standard
• Clocks tick at constant rate regardless of season
• "Clock time" replaced "sun time" for daily schedules
• Enabled precise coordination of activities

Social impact:

• Time discipline: Workers expected at specific times
• Urban life required synchronization
• "Punctuality" became a virtue
• Transition from natural rhythms to mechanical rhythms

Scientific Definition (1800s)

Astronomical measurement:

• 1832: Second officially defined as 1/86,400 of mean solar day
• Astronomers recognized Earth's rotation not perfectly uniform
• Tidal friction slowly increases day length (~1.7 milliseconds per century)

Problem discovered:

• Earth's rotation varies:
  • Seasonal variations (atmosphere, ice melt)
  • Long-term slowing (tidal friction from Moon)
  • Irregular variations (core-mantle coupling, earthquakes)
• "Day" based on Earth rotation became unreliable time standard

Atomic Era: Day Decoupled from Rotation (1967)

Atomic second (1967):

• Second redefined based on caesium-133 atomic transitions
• Day remains 86,400 seconds (by definition)
• But now independent of Earth's actual rotation period

Consequence: Leap seconds

• Earth's rotation gradually slowing
• Atomic time (TAI) and Earth rotation time (UT1) drift apart
• Leap seconds added to keep them synchronized:
  • 27 leap seconds added between 1972-2016
  • Last one: December 31, 2016 (23:59:60)
  • Makes that day 86,401 seconds long
• Controversy: May abolish leap seconds in favor of "leap hours" every few centuries

Current system:

• UTC (Coordinated Universal Time): Atomic time with leap seconds
• Keeps within 0.9 seconds of Earth rotation (UT1)
• Used for civil timekeeping worldwide

Calendar Evolution

Ancient calendars:

• Lunar calendars: Based on moon phases (~29.5 days per month)
• Solar calendars: Based on seasonal year (365.25 days)
• Lunisolar calendars: Combine both (Hebrew, Chinese)

Gregorian Calendar (1582):

• Reformed Julian calendar
• Year = 365.2425 days (very close to true solar year: 365.2422 days)
• Leap year rules:
  • Divisible by 4: Leap year (1600, 2000, 2004, 2024)
  • Divisible by 100: Not leap year (1700, 1800, 1900)
  • Divisible by 400: Leap year anyway (1600, 2000, 2400)
• Now used in nearly all countries for civil purposes

Real-World Examples

1. Human Biology and Circadian Rhythms

The day is hardwired into human biology:

• Circadian clock:

  • ~24-hour internal biological rhythm
  • Controlled by suprachiasmatic nucleus (SCN) in hypothalamus
  • Regulated by light exposure (especially blue light)
  • Controls sleep-wake cycle, hormone release, body temperature

• Daily hormonal patterns:

  • Cortisol: Peaks 8-9 AM (waking hormone, highest alertness)
  • Melatonin: Rises ~9 PM, peaks 2-4 AM (sleep hormone)
  • Growth hormone: Released during deep sleep (first 3-4 hours)
  • Body temperature: Lowest 4-5 AM, highest 6-7 PM

• Sleep-wake cycle:

  • Humans naturally sleep ~7-9 hours per 24-hour day
  • Without time cues, humans drift to ~25-hour cycle (but entrain to 24-hour day with light)
  • Shift work disrupts circadian rhythm → health problems

• Jet lag:

  • Crossing time zones faster than body can adjust
  • ~1 day recovery per 1-hour time zone crossed
  • Eastward travel harder (phase advance) than westward (phase delay)

• Seasonal affective disorder (SAD):

  • Depression linked to shorter daylight hours in winter
  • Treated with bright light therapy (mimicking longer days)

2. Work and Employment

The day structures modern work life:

• Workday:

  • Standard: 8 hours per day, 5 days per week
  • "9-to-5": Typical business hours (9 AM - 5 PM)
  • "Business day": Monday-Friday, excluding weekends/holidays

• Shift work:

  • Day shift: 7 AM - 3 PM or 8 AM - 4 PM
  • Evening shift: 3 PM - 11 PM or 4 PM - midnight
  • Night shift: 11 PM - 7 AM or midnight - 8 AM
  • Rotating shifts: Change every few days or weeks

• Daily wage:

  • Some workers paid per day worked
  • "Day laborer": Hired by the day
  • "Per diem": Daily allowance for expenses

• Time off:

  • Sick day: One day absence due to illness
  • Vacation day: One day of paid time off
  • Personal day: One day off for personal matters
  • Accumulated as "days per year" (e.g., 15 vacation days/year)

• Deadlines:

  • "Due by end of business day" (typically 5 PM)
  • "Two-day turnaround"
  • "30-day notice"

3. Calendar and Dating Systems

Days form the basis of all calendars:

• Date format:

  • ISO 8601 standard: YYYY-MM-DD (2024-11-26)
  • US format: MM/DD/YYYY (11/26/2024)
  • European format: DD/MM/YYYY (26/11/2024)

• Days of the week (7-day cycle):

  • Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday
  • Named after celestial bodies/Norse gods in English
  • Universal globally (with name variations)

• Weekday vs. weekend:

  • Weekdays: Monday-Friday (work/school days)
  • Weekend: Saturday-Sunday (rest days in Western tradition)
  • Varies by culture: Friday-Saturday in Muslim countries, Sunday only in some regions

• Special day designations:

  • Business days: Monday-Friday, excluding holidays
  • Calendar days: Every day including weekends/holidays
  • "Ships in 3-5 business days" vs. "Valid for 30 calendar days"

• Holidays:

  • Fixed date: December 25 (Christmas), July 4 (US Independence Day)
  • Floating: Thanksgiving (4th Thursday of November), Easter (varies)
  • Different countries: Over 100 different national holidays worldwide

• Day counting:

  • "Day 1" vs. "Day 0": Ambiguity in counting
  • Age calculation: Count completed days (birthday anniversary)
  • Pregnancy: Counted in weeks (280 days = 40 weeks)

4. Travel and Accommodation

Days measure travel duration and lodging:

• Hotel/rental pricing:

  • Charged per "night" = one day stay
  • Check-in: Typically 3-4 PM
  • Check-out: Typically 10-11 AM
  • "Day use" rate: Use room during daytime hours only

• Vacation length:

  • Weekend trip: 2-3 days
  • Week-long vacation: 7 days
  • "10-day tour" packages

• Travel duration:

  • "3-day road trip"
  • "2-day train journey"
  • Ocean voyage: Measured in days (7-day cruise)

• Visas and permits:

  • Tourist visa: "30 days," "90 days," "180 days"
  • Parking permit: "Valid for 1 day"
  • Visa overstay: Calculated in days past expiration

• Advance booking:

  • "Book 14 days in advance for best rates"
  • "Cancellation must be 48 hours (2 days) before arrival"

5. Food and Perishables

Freshness and safety measured in days:

• Expiration dates:

  • "Best by" date: Quality guarantee
  • "Use by" date: Safety deadline
  • "Sell by" date: Store inventory guide

• Food freshness:

  • Milk: 5-7 days after opening
  • Cooked meat: 3-4 days refrigerated
  • Fresh fish: 1-2 days
  • Bread: 5-7 days at room temperature

• Fasting and diets:

  • Intermittent fasting: "Fast every other day" (alternate-day fasting)
  • "5:2 diet": Normal eating 5 days, restricted 2 days
  • Religious fasts: Ramadan (30 days), Lent (40 days)

• Aging processes:

  • Cheese aging: 60 days (cheddar), 180+ days (parmesan)
  • Wine aging: 180-365+ days in barrel
  • Dry-aged beef: 21-28 days

6. Medication and Healthcare

Medical treatments often measured in days:

• Prescription duration:

  • Antibiotics: "10-day course"
  • "Take for 7 days"
  • "30-day supply"

• Hospital stays:

  • Average stay: 4-5 days (US hospitals)
  • ICU stay: Measured in days
  • Recovery time: "7-10 days rest"

• Quarantine/isolation:

  • COVID-19: 5-10 days isolation
  • Chickenpox: 5-7 days contagious
  • Exposure quarantine: 14 days (2 weeks)

• Menstrual cycle:

  • ~28-day cycle average
  • Period: 3-7 days
  • Ovulation: Day 14 (typical)

• Pregnancy:

  • 280 days from last menstrual period (40 weeks)
  • Trimesters: ~90 days each
  • "Due date" calculated in days

• Drug half-life:

  • "Remains in system for 3 days"
  • Withdrawal symptoms: "Peak at 2-3 days"

7. Legal and Contractual

Legal timeframes measured in days:

• Notice periods:

  • Job resignation: 14 days (2 weeks), 30 days (1 month)
  • Eviction notice: 30-60 days depending on jurisdiction
  • Contract termination: "30-day notice required"

• Statute of limitations:

  • Traffic tickets: 30 days to pay/contest
  • Small claims: 15-30 days to respond
  • Credit card dispute: 60 days to report fraud

• Waiting periods:

  • Marriage license: 1-3 days waiting period (some states)
  • Gun purchase: 3-10 days (varies by state)
  • Check hold: 2-7 business days

• Trial periods:

  • Money-back guarantee: "30-day return policy"
  • Free trial: "7-day trial"
  • Probation period: 90 days (3 months)

• Grace periods:

  • Bill payment: 10-15 days grace period
  • Insurance: 30-day grace period
  • "Due within 30 days"

Common Uses and Applications

1. Age and Lifespan Measurement

Human life measured in days:

• Age calculation:

  • Newborn: Age in days (first month)
  • Infant: Days and weeks (first 12 months)
  • Adult: Years (365.25 days per year)

• Life expectancy:

  • Global average: ~73 years = 26,645 days
  • US average: ~78 years = 28,470 days
  • Japan (highest): ~84 years = 30,660 days

• Milestones:

  • 100 days: Traditional celebration in some cultures
  • 1,000 days: ~2.7 years (toddler milestone)
  • 10,000 days: ~27.4 years (young adult)
  • 20,000 days: ~54.8 years (mid-life)
  • 30,000 days: ~82.2 years (if reached, long life)

• Historical figures:

  • "Lived 90 years" = 32,850 days
  • Queen Elizabeth II: 35,065 days (96 years, 140 days)
  • Oldest verified person: Jeanne Calment, 44,724 days (122 years, 164 days)

2. Project Management and Planning

Projects measured in days:

• Timeline terminology:

  • "Day 0": Project start
  • "Elapsed days": Total calendar days
  • "Working days": Excluding weekends/holidays
  • "Man-days": One person working one day

• Estimation:

  • "3-day task"
  • "2-week project" = 10 working days
  • "6-month project" = ~130 working days

• Milestones:

  • "Deliverable due Day 30"
  • "Phase 1 complete Day 45"
  • "Final deadline Day 90"

• Agile/Scrum:

  • Sprint: 14 days (2 weeks) typical
  • Daily standup: Every day, 15 minutes
  • Sprint review: End of 14-day sprint

3. Astronomy and Planetary Science

Planetary rotation periods measured in days:

• Planetary "days" (rotation period):

  • Mercury: 58.6 Earth days
  • Venus: 243 Earth days (slower than its year!)
  • Earth: 1 day (23 hours 56 min sidereal)
  • Mars: 1.03 days (24 hours 37 min) - called a "sol"
  • Jupiter: 0.41 days (9 hours 56 min)
  • Saturn: 0.45 days (10 hours 33 min)
  • Uranus: 0.72 days (17 hours 14 min)
  • Neptune: 0.67 days (16 hours 6 min)

• Orbital periods (years in days):

  • Mercury year: 88 Earth days
  • Venus year: 225 Earth days
  • Mars year: 687 Earth days
  • Earth year: 365.25 days

• Mars missions:

  • Use "sols" (Mars days) for mission planning
  • Sol 1, Sol 2, Sol 3... (rovers like Curiosity, Perseverance)
  • Communication delay: 3-22 minutes (depends on planets' positions)

• Astronomical events:

  • Lunar month: 29.53 days (new moon to new moon)
  • Eclipse cycles: Saros cycle = 6,585.3 days (18 years, 11 days)

4. Weather and Climate

Weather patterns measured in days:

• Forecasting:

  • 1-day forecast: Very accurate (~90%)
  • 3-day forecast: Accurate (~80%)
  • 7-day forecast: Moderately accurate (~65%)
  • 10+ day forecast: Less reliable

• Weather phenomena:

  • Heat wave: 3+ consecutive days above threshold
  • Cold snap: 2+ days below freezing
  • Drought: 15+ days without significant rain

• Seasonal patterns:

  • Growing season: Number of frost-free days (150-200+ days)
  • Rainy season: 90-180 days (tropics)
  • Winter: Shortest day (winter solstice) vs. longest night

• Degree days:

  • Heating degree days (HDD): Measure of cold
  • Cooling degree days (CDD): Measure of heat
  • Base 65°F: Sum of daily degrees below/above

• Climate records:

  • "Hottest day on record"
  • "100 days above 90°F" (Phoenix averages 110+ days)
  • "Consecutive days of rain" (record: 331 days, Kauai)

5. Finance and Business

Financial operations measured in days:

• Payment terms:

  • Net 30: Payment due 30 days after invoice
  • Net 60: Payment due 60 days after invoice
  • 2/10 Net 30: 2% discount if paid within 10 days, otherwise due in 30

• Interest calculation:

  • Daily interest: Annual rate ÷ 365 days
  • Grace period: 21-25 days (credit cards)
  • Late fees: Applied after due date + grace period

• Financial metrics:

  • Days sales outstanding (DSO): Average days to collect payment
  • Days payable outstanding (DPO): Average days to pay suppliers
  • Days inventory outstanding (DIO): Average days inventory held

• Trading:

  • "Trading day": Stock market open day (weekdays, excluding holidays)
  • NYSE: ~252 trading days per year
  • Settlement: T+2 (trade day + 2 business days)

• Bonds:

  • Accrued interest calculated by day
  • 30/360 day count convention (assumes 30-day months)
  • Actual/365: Uses actual calendar days

6. Data Storage and Computing

Digital retention measured in days:

• Backups:

  • Daily backups: 7 days retained (1 week)
  • Weekly backups: 30 days retained (1 month)
  • Monthly backups: 365 days retained (1 year)

• Logs:

  • Server logs: 30-90 days retention typical
  • Security logs: 90-365 days (compliance requirements)
  • Application logs: 14-30 days

• Caching:

  • Browser cache: 30 days default
  • CDN cache: 1-30 days depending on content
  • DNS cache: 1 day (86,400 seconds TTL common)

• Data retention policies:

  • GDPR: 30 days to fulfill deletion request
  • Email: Auto-delete after 90 days (some organizations)
  • Trash/recycle bin: 30 days before permanent deletion

7. Habits and Personal Development

Habit formation measured in days:

• Popular beliefs:

  • "21 days to form a habit" (myth - actually varies widely)
  • "30-day challenge" (fitness, meditation, etc.)
  • "90-day transformation programs"

• Research findings:

  • Average habit formation: 66 days (range: 18-254 days)
  • Simple habits: 18-30 days
  • Complex habits: 200+ days

• Streaks:

  • "100-day streak" on language apps (Duolingo)
  • "30-day yoga challenge"
  • "365-day photo project" (one photo per day for a year)

• Reading goals:

  • "Read every day for 30 days"
  • "One book per week" = finish in 7 days
  • "365 books in a year" = 1 per day

Conversion Guide

Basic Day Conversions

Days to hours:

• Formula: hours = days × 24
• Example: 2.5 days = 2.5 × 24 = 60 hours

Hours to days:

• Formula: days = hours ÷ 24
• Example: 72 hours = 72 ÷ 24 = 3 days

Days to minutes:

• Formula: minutes = days × 1,440
• Example: 1 day = 1 × 1,440 = 1,440 minutes

Minutes to days:

• Formula: days = minutes ÷ 1,440
• Example: 2,880 minutes = 2,880 ÷ 1,440 = 2 days

Days to seconds:

• Formula: seconds = days × 86,400
• Example: 1 day = 1 × 86,400 = 86,400 seconds

Seconds to days:

• Formula: days = seconds ÷ 86,400
• Example: 259,200 seconds = 259,200 ÷ 86,400 = 3 days

Days to weeks:

• Formula: weeks = days ÷ 7
• Example: 14 days = 14 ÷ 7 = 2 weeks

Weeks to days:

• Formula: days = weeks × 7
• Example: 4 weeks = 4 × 7 = 28 days

Days to years (approximate):

• Formula: years = days ÷ 365.25 (accounts for leap years)
• Example: 730 days = 730 ÷ 365.25 = ~2 years

Years to days (approximate):

• Formula: days = years × 365.25
• Example: 5 years = 5 × 365.25 = ~1,826 days

Comprehensive Conversion Table

Days	Hours	Minutes	Seconds	Weeks	Months (30-day)	Years
1	24	1,440	86,400	0.143	0.033	0.0027
7	168	10,080	604,800	1	0.233	0.019
14	336	20,160	1,209,600	2	0.467	0.038
30	720	43,200	2,592,000	4.286	1	0.082
60	1,440	86,400	5,184,000	8.571	2	0.164
90	2,160	129,600	7,776,000	12.857	3	0.246
180	4,320	259,200	15,552,000	25.714	6	0.493
365	8,760	525,600	31,536,000	52.143	12.167	1
366	8,784	527,040	31,622,400	52.286	12.2	1.003
1,000	24,000	1,440,000	86,400,000	142.857	33.333	2.738
10,000	240,000	14,400,000	864,000,000	1,428.571	333.333	27.379

Common Time Period Conversions

Period	Days	Notes
Week	7	Universal
Fortnight	14	2 weeks
Lunar month	29.53	New moon to new moon
February (non-leap)	28	Shortest month
February (leap year)	29	Every 4 years (with exceptions)
April, June, Sept, Nov	30	"30 days hath September..."
Jan, Mar, May, Jul, Aug, Oct, Dec	31	Longest months
Quarter (business)	90-92	~3 months
Trimester	90-93	~3 months (pregnancy: 280 days total)
Year (common)	365	Most years
Year (leap)	366	Divisible by 4 (exceptions for centuries)
Solar year (tropical)	365.2422	Earth's orbit around sun
Sidereal year	365.2564	Relative to fixed stars
Olympiad	1,461	4 years (365.25 × 4)
Decade	3,652.5	10 years (accounting for leap years)
Generation	~10,957	~30 years
Century	36,524.25	100 years (accounting for leap year rules)

Business Day Conversions

US business days (excluding weekends and ~10 federal holidays):

• ~252 trading days per year (stock market)
• ~260 working days per year (typical employment)

Period	Calendar Days	Business Days (approx)
1 week	7	5
2 weeks	14	10
1 month	30	21-22
1 quarter	90	63-64
1 year	365	260

Calculating business days:

• Exclude weekends: (Total days ÷ 7) × 5
• Adjust for holidays: Subtract ~2 per month
• Example: 30 calendar days ≈ (30 ÷ 7) × 5 = 21.4 ≈ 21 business days

Age in Days

Age	Approximate Days	Exact Calculation
1 year	365	365 or 366 (leap year)
5 years	1,826	5 × 365.25 = 1,826.25
10 years	3,653	10 × 365.25 = 3,652.5
18 years	6,575	18 × 365.25 = 6,574.5
21 years	7,671	21 × 365.25 = 7,670.25
30 years	10,958	30 × 365.25 = 10,957.5
50 years	18,263	50 × 365.25 = 18,262.5
75 years	27,394	75 × 365.25 = 27,393.75
100 years	36,525	100 × 365.25 = 36,525

Note: Exact age in days requires accounting for actual leap years experienced.

Common Conversion Mistakes

1. Assuming All Months Have 30 Days

WRONG: "90 days = 3 months exactly" CORRECT: 90 days ≈ 2.96-3 months (depending on which months)

Why this varies:

• February: 28-29 days
• April, June, September, November: 30 days
• January, March, May, July, August, October, December: 31 days

Examples:

• January 1 to March 31 = 90 days (31 + 28 + 31) - non-leap year
• January 1 to March 31 = 91 days (31 + 29 + 31) - leap year
• April 1 to June 30 = 91 days (30 + 31 + 30)

Safer: Use specific dates or say "approximately 3 months"

2. Business Days vs. Calendar Days

WRONG: "Arrives in 7 business days" = 7 calendar days CORRECT: 7 business days ≈ 9-10 calendar days (includes weekend)

Calculation:

• Order Friday: 7 business days = following Tuesday
• Friday (day 0) → Monday (1), Tuesday (2), Wednesday (3), Thursday (4), Friday (5), Monday (6), Tuesday (7)
• That's 11 calendar days including the weekend

Common errors:

• Shipping estimates: "3-5 business days" often surprises customers expecting 3-5 calendar days
• Contract deadlines: "30 business days" ≠ "30 calendar days" (can be 42+ calendar days)

Pro tip: Always clarify "business days" vs. "calendar days" in agreements

3. Leap Year Confusion

WRONG: "Every 4 years is a leap year" CORRECT: Divisible by 4, except centuries unless divisible by 400

Leap year rules:

1. If divisible by 4 → leap year (2024, 2028)
2. EXCEPT if divisible by 100 → not leap year (1900, 2100)
3. UNLESS divisible by 400 → leap year anyway (2000, 2400)

Examples:

• 2000: Leap year (divisible by 400) ✓
• 1900: Not leap year (divisible by 100, not by 400) ✗
• 2024: Leap year (divisible by 4) ✓
• 2100: Not leap year (divisible by 100, not by 400) ✗

Impact on age calculations:

• 100 years ≠ exactly 36,500 days
• 100 years = 36,524 or 36,525 days (depending on which 100 years)

4. Day Counting Ambiguity ("Inclusive" vs. "Exclusive")

WRONG: Friday to Monday = 4 days DEPENDS ON COUNTING METHOD:

• Inclusive (counting both endpoints): Friday, Saturday, Sunday, Monday = 4 days
• Exclusive (duration): Friday to Monday = 3 days elapsed

Common scenarios:

• Hotel stays: "Check in Friday, check out Monday" = 3 nights (not 4)
• Event duration: "Conference runs Thursday-Saturday" = 3 days (inclusive)
• Age: "3 days old" = born 3 days ago (exclusive counting)

Clearer phrasing:

• "Friday through Monday" (inclusive: 4 days)
• "From Friday to Monday" (ambiguous)
• "Friday to Monday, 3 nights" (exclusive: duration)

5. Years to Days Conversion Errors

WRONG: 1 year = 365 days (always) CORRECT: 1 year = 365 or 366 days (depending on leap year)

Better approximation:

• 365.25 days per year (accounts for leap years)
• Over 4 years: 365 + 365 + 365 + 366 = 1,461 days ÷ 4 = 365.25 average

Even more accurate:

• 365.2425 days per year (Gregorian calendar average)
• Accounts for century exception rule

Examples:

• 10 years ≈ 3,652.5 days (using 365.25)
• 100 years ≈ 36,524.25 days (using 365.2425)

When it matters:

• Age calculations over many years
• Astronomical calculations
• Long-term project planning

6. Time Zone Day Boundary Issues

WRONG: "It's Monday everywhere at midnight UTC" CORRECT: Different time zones experience Monday at different times

Reality:

• When it's Monday 00:00 in London (UTC), it's:
  • Still Sunday 19:00 (7 PM) in New York (UTC-5)
  • Still Sunday 16:00 (4 PM) in Los Angeles (UTC-8)
  • Already Monday 09:00 (9 AM) in Tokyo (UTC+9)

Implications:

• International contract deadlines: "Due Monday" = which time zone?
• Global events: "New Year's Day" celebrated across 26 hours (UTC-12 to UTC+14)
• Space station: ISS uses UTC time (crew experiences 16 sunrises per day!)

Pro tip: Always specify time zone for day-based deadlines