Day to Fortnight Converter

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)

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

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