Week (wk) - Unit Information & Conversion

Quick Answer

A week is 7 days, 168 hours, or 10,080 minutes—the universal cycle for organizing work, rest, and social life worldwide. From Monday through Sunday (or Sunday through Saturday in some cultures), the week structures everything from paychecks to worship days. The name "week" comes from Old English wice, related to wikan (to yield/change), reflecting the weekly rotation of activities.

Real-world perspective: One week is a pay period for many workers, the time between two Mondays, a standard vacation unit ("two-week trip"), the cycle from Sunday to Sunday, or the planning horizon for most people's schedules.

Quick Comparison Table

Time Period	Weeks	Days	Hours	Minutes
One week	1	7	168	10,080
Fortnight	2	14	336	20,160
Three weeks	3	21	504	30,240
Month (avg)	~4.35	30	720	43,200
Quarter	13	91	2,184	131,040
Year	52.14	365	8,760	525,600
Leap year	52.29	366	8,784	527,040
Decade	521.4	3,652.5	87,660	5,259,600

Definition

The week (symbol: wk or w) is a unit of time equal to 7 days, 168 hours, or 10,080 minutes.

Official status: The week is not an SI unit, but it is accepted for use with the SI due to its universal cultural importance. The SI base unit of time is the second, and the day is the fundamental accepted non-SI unit.

Standard conversions:

• 1 week = 7 days (exact)
• 1 week = 168 hours (7 × 24)
• 1 week = 10,080 minutes (7 × 24 × 60)
• 1 week = 604,800 seconds (7 × 24 × 60 × 60)
• 1 year ≈ 52.14 weeks (365 ÷ 7)
• 1 month ≈ 4.35 weeks (30 ÷ 7)

The 7-day structure: The week consists of seven consecutive days, typically organized as:

International (Monday-first) convention:

1. Monday (Moon's day) - Start of work week
2. Tuesday (Tiw's day, Norse god of war)
3. Wednesday (Woden's day, Odin)
4. Thursday (Thor's day, god of thunder)
5. Friday (Frigg's day, goddess of love)
6. Saturday (Saturn's day)
7. Sunday (Sun's day) - Traditional day of rest

US (Sunday-first) convention:

• Sunday considered first day of the week on US calendars
• Work week runs Monday-Friday
• Weekend is Saturday-Sunday

ISO 8601 standard:

• Monday is officially day 1 of the week
• Sunday is day 7
• Week numbering: Week 1 contains first Thursday of year

Workweek vs. weekend:

• Workweek/weekdays: Monday-Friday (5 days) in Western tradition
• Weekend: Saturday-Sunday (2 days) in Western tradition
• Varies by culture: Friday-Saturday in Muslim countries, Sunday only historically

Why 7 days, not 5, 8, or 10? Unlike the day (Earth rotation) or year (orbital period), the week has no astronomical basis. It's purely a human cultural construct that gained universal adoption through:

1. Ancient Babylonian astronomy (7 visible celestial bodies)
2. Jewish religious tradition (Genesis creation, Sabbath commandment)
3. Christian adoption and spread (Sunday worship)
4. Islamic adoption (Friday as holy day)
5. Roman Empire standardization (321 CE Constantine decree)
6. Deep cultural entrenchment making change impractical

History of the Week

Ancient Babylonian Origins (c. 2000-1000 BCE)

The 7-day week's roots lie in ancient Mesopotamian astronomy and astrology:

Babylonian astronomy:

• Observed seven "wandering stars" (planets) visible to naked eye:

  1. Sun (Shamash) - brightest object
  2. Moon (Sin) - most obviously changing
  3. Mercury (Nabu) - messenger god
  4. Venus (Ishtar) - morning/evening star
  5. Mars (Nergal) - red planet, war god
  6. Jupiter (Marduk) - king of gods
  7. Saturn (Ninurta) - slow-moving

• Each celestial body "ruled" one day

• Seven was considered mystical/sacred number

• Used in astrological predictions and religious rituals

Why 7 was special:

• Seven visible "planets" (including Sun and Moon)
• Seven days between moon phases (~7.4 days per quarter)
• Mathematical: 7 is prime, making it special
• Religious significance in Near Eastern cultures

Note: The moon's phases (29.5 days ÷ 4 ≈ 7.4 days) may have influenced the 7-day cycle, though it doesn't align perfectly.

Jewish Religious Codification (c. 1500-500 BCE)

The Hebrew Bible (Torah) embedded the 7-day week in religious law:

Genesis creation narrative (Genesis 1:1-2:3):

• Day 1: Light and darkness
• Day 2: Sky and waters
• Day 3: Land, seas, plants
• Day 4: Sun, moon, stars
• Day 5: Fish and birds
• Day 6: Land animals and humans
• Day 7: God rested → Sabbath (Shabbat)

Fourth Commandment (Exodus 20:8-11):

"Remember the Sabbath day, to keep it holy. Six days you shall labor and do all your work, but the seventh day is a Sabbath to the Lord your God."

Sabbath observance:

• Saturday (7th day) as mandatory day of rest
• No work permitted (cooking, travel, commerce)
• Synagogue worship and family meals
• Violations carried severe penalties (death in ancient times)
• Core to Jewish identity for 3,000+ years

Jewish week structure:

• Days numbered: Yom Rishon (Day 1) through Yom Shishi (Day 6)
• Only Shabbat (Sabbath, Day 7) has a name
• Week begins Saturday evening (sunset) and ends following Saturday sunset

Greek and Roman Adoption (300 BCE - 400 CE)

Greek influence:

• Hellenistic astronomers (post-Alexander) adopted Babylonian astrology
• Each day associated with a planet/deity
• Week spread through Greek-speaking world
• Ptolemy's astrology (2nd century CE) codified planetary hours and days

Roman nundinal cycle (753 BCE - 321 CE):

• Romans initially used 8-day market week (nundinae)
• Days labeled A through H
• Markets held every 8th day
• Used for agricultural and commercial scheduling

Planetary week adoption (1st-3rd century CE):

• 7-day planetary week entered Rome from Near East
• Coexisted with 8-day nundinal cycle
• Gradually replaced nundinal week for religious/astrological reasons
• Days named after planets/gods:
  • Dies Solis (Sun) → Sunday
  • Dies Lunae (Moon) → Monday
  • Dies Martis (Mars) → Tuesday (Tiw = Germanic Mars)
  • Dies Mercurii (Mercury) → Wednesday (Woden = Germanic Mercury)
  • Dies Jovis (Jupiter) → Thursday (Thor = Germanic Jupiter)
  • Dies Veneris (Venus) → Friday (Frigg = Germanic Venus)
  • Dies Saturni (Saturn) → Saturday

Constantine's decree (321 CE):

• Emperor Constantine I officially recognized the 7-day week
• Declared Sunday (Dies Solis) a day of rest
• Aligned with Christian practice (resurrection day)
• Marked official end of nundinal cycle
• Made 7-day week legal standard across Roman Empire

Christian Transformation (1st-5th century CE)

Early Christian practice:

• Jewish Christians initially observed Saturday Sabbath
• Gradually shifted to Sunday (Dies Dominica, "Lord's Day")
• Commemorated Jesus's resurrection (Sunday morning)
• Sunday worship established by 100 CE

Christian week structure:

• Sunday: Lord's Day, primary worship
• Monday-Saturday: Workdays
• No Sabbath work prohibition (unlike Judaism)
• Sunday rest became custom, not religious law initially

Church influence:

• Constantine's decree (321 CE) made Sunday official rest day
• Christian terminology replaced pagan planet names in some languages:
  • Portuguese: Domingo (Sunday = Lord's Day), Segunda-feira (Monday = Second day)
  • Some Slavic languages: similar pattern
• Christian calendar organized around Sunday as "first day of week" (Western tradition)

Medieval Christian week:

• Elaborate liturgical calendar
• Different saints' days on specific weekdays
• Friday fasting (commemorating crucifixion)
• Sunday mandatory Mass attendance
• Week structured around religious observances

Islamic Adoption (7th century CE)

Islamic week (al-usbūʿ):

• Adopted existing 7-day week structure
• Friday (Jumu'ah) designated as day of congregational prayer
• Not a "day of rest" like Sabbath/Sunday—work permitted
• Friday midday prayer (Jumu'ah prayer) mandatory for men

Islamic day names:

• Days numbered similar to Hebrew tradition
• Saturday: Yawm as-Sabt (Day of the Sabbath—Hebrew influence)
• Sunday: Yawm al-Ahad (First day)
• Monday: Yawm al-Ithnayn (Second day)
• ...
• Friday: Yawm al-Jumu'ah (Day of Congregation)

Spread of Islamic week:

• Islamic expansion (7th-15th centuries) spread 7-day week to:
  • North Africa
  • Middle East
  • Central Asia
  • Parts of Southeast Asia
• Reinforced 7-day week as global standard

Global Standardization (1500-1900)

European colonialism:

• Spanish, Portuguese, French, British empires spread 7-day week
• Christian Sunday observance imposed in colonies
• Replaced indigenous time-keeping systems:
  • Aztec 13-day and 20-day cycles
  • Mayan complex calendar system
  • Various Asian lunar-based systems

East Asia adoption:

• China: Adopted 7-day week in early 20th century (previously used 10-day xún divisions)
• Japan: Officially adopted 7-day week in 1873 during Meiji Restoration
• Korea: Adopted with modernization in late 19th/early 20th century

International commerce:

• Global trade required synchronized schedules
• Shipping and maritime schedules used 7-day week
• Telegraph and later telecommunications standardized weekly communications

Failed Reform Attempts

Despite universal adoption, several attempts to "improve" the week failed:

1. French Revolutionary Calendar (1793-1805):

• Replaced 7-day week with 10-day décade
• Aligned with metric system (10 days per week, 3 weeks per month)
• Days numbered Primidi through Décadi
• Only Décadi was rest day (1 in 10 vs. 1 in 7)
• Failed because:
  • Less frequent rest days unpopular with workers
  • Conflicted with Christian Sunday observance
  • Disrupted social and family patterns
  • Napoleon abolished it in 1805

2. Soviet 5-day and 6-day weeks (1929-1940):

• 1929-1931: 5-day "continuous week"

  • Days numbered 1-5
  • Each worker got one of five days off (rotating)
  • Goal: Continuous factory production
  • Problem: Families/friends couldn't synchronize time off

• 1931-1940: 6-day week

  • Days numbered 1-6
  • Day 6 was universal rest day
  • Goal: Improve on 5-day system
  • Problem: Still disrupted religious observance, traditional patterns

• 1940: Return to 7-day week

  • Abandoned experiments
  • Restored traditional Sunday rest
  • 7-day week too culturally embedded to change

3. International Fixed Calendar (1923-present, never adopted):

• Proposed by Moses B. Cotsworth
• 13 months of 28 days each (4 perfect weeks per month)
• Extra month called "Sol" between June and July
• One "Year Day" outside the weekly cycle
• Never adopted because:
  • Would disrupt all existing calendars
  • Breaking the continuous 7-day cycle unacceptable religiously
  • Massive economic costs
  • Resistance from established institutions

4. Other proposals:

• Decimal weeks (10 days)
• 5-day weeks (aligned with work week)
• 8-day weeks (better divides into month)
• All failed: Cultural inertia too strong

Modern Universal Adoption

Current status:

• All 195+ countries use the 7-day week
• Synchronized globally despite cultural differences
• ISO 8601 standard (Monday = day 1, week 1 contains first Thursday)
• Different weekend patterns:
  • Saturday-Sunday: Most of world (Christian tradition)
  • Friday-Saturday: Many Muslim countries (Saudi Arabia, UAE until 2022)
  • Friday only: Iran
  • Sunday only: Historical in some countries

Why 7-day week succeeded:

1. Religious universality: Judaism, Christianity, Islam all use 7-day week
2. Ancient origins: 3,000+ years of continuity
3. Global colonization: European powers spread it worldwide
4. Economic integration: International commerce requires synchronization
5. Cultural entrenchment: Too deeply embedded to change
6. Mathematical convenience: Fits reasonably with months (4-5 weeks)
7. Work-rest balance: 5-2 or 6-1 work-rest ratio culturally accepted

Modern cultural significance:

• Phrase "work week" universal
• "Weekend" concept global (even if different days)
• Weekly planning horizon standard
• Pay periods often weekly or bi-weekly
• Television programming on weekly schedules
• Religious observances every 7 days
• Social rhythms organized weekly

Real-World Examples

1. Work and Employment

The week structures modern employment:

• Standard work week:

  • US/Western: Monday-Friday, 40 hours (5 days × 8 hours)
  • Part-time: 20-30 hours per week
  • Full-time: 35-40 hours per week (varies by country)
  • France: 35-hour work week (legal standard)
  • Denmark: 37-hour work week average

• Pay periods:

  • Weekly: Paid every week (common for hourly workers)
  • Bi-weekly: Paid every 2 weeks (26 pay periods per year)
  • Semi-monthly: Paid twice per month (24 pay periods per year)
  • Weekly pay often Friday (end of work week)

• Weekend:

  • Saturday-Sunday: Western standard (2 days off)
  • Friday-Saturday: Muslim countries (Saudi Arabia, UAE historically)
  • Sunday only: Historical European/American standard (now rare)
  • "Thank God It's Friday" (TGIF) cultural phenomenon

• Work schedules:

  • "9-to-5, Monday-Friday" - archetypal Western work week
  • Flexible work: 4-day work week experiments (32-40 hours over 4 days)
  • Shift work: Rotating weekly schedules
  • Weekend work: Premium pay (time-and-a-half or double-time)

• Unemployment benefits:

  • "Weeks of eligibility" (e.g., "26 weeks of unemployment insurance")
  • Job search requirements: "2-3 applications per week"

• Work-life balance metrics:

  • "Work-life balance" measured in hours per week worked
  • Overwork: 50-60+ hours per week
  • Burnout linked to excessive weekly hours

2. Education and Academia

Schools and universities organized by weeks:

• School year:

  • US K-12: 36-40 weeks of instruction per year
  • University semester: 15-16 weeks
  • Quarter system: 10-11 weeks
  • Summer school: 6-8 weeks

• School week:

  • Monday-Friday: Standard school days
  • 5 days × 6-8 hours = 30-40 hours per week
  • Weekend homework: 2-5 hours per week (varies by grade)

• Academic calendar:

  • "Week 1" through "Week 15" course schedules
  • Midterms: Week 7-8
  • Finals: Week 15-16
  • Reading week/Spring break: Full week off

• Assignment deadlines:

  • "Due next week"
  • "Two-week project"
  • "Weekly homework"

• College workload:

  • Rule of thumb: 2-3 hours study per 1 hour class per week
  • 15 credit hours = 15 hours class + 30-45 hours study = 45-60 hours/week total

3. Health and Fitness

Exercise and health measured weekly:

• Exercise recommendations:

  • WHO: 150 minutes moderate exercise per week (adults)
  • CDC: Same, equals ~30 minutes × 5 days per week
  • Strength training: 2-3 days per week
  • Running: 20-50 miles per week (serious runners)

• Weight loss:

  • Healthy goal: 1-2 pounds per week
  • Measured weekly on same day (e.g., "Weigh-in Wednesdays")
  • Calorie deficit: 3,500 calories per week = 1 pound lost

• Meal prep:

  • "Weekly meal prep" on Sundays
  • Planning 21 meals per week (3 per day × 7 days)
  • Grocery shopping: Weekly trip

• Medication schedules:

  • "Once weekly" medications (e.g., some osteoporosis drugs)
  • Pill organizers labeled by day of week
  • Birth control: 3-week active + 1-week placebo

• Health tracking:

  • "Steps per week" (7,000-10,000 steps/day × 7)
  • "Active minutes per week"
  • "Sleep hours per week" (56 hours ideal = 8 hr/night × 7)

4. Finance and Business

Financial cycles measured in weeks:

• Payroll:

  • Weekly payroll: 52 paychecks per year
  • Bi-weekly: 26 paychecks per year
  • Weekly cash flow management for hourly workers

• Business metrics:

  • Sales: "This week vs. last week" comparisons
  • Traffic: "Weekly active users" (WAU) in tech
  • Revenue: "Revenue per week"
  • Inventory: "Weeks of supply" (inventory ÷ weekly sales)

• Stock market:

  • Trading week: Monday-Friday (5 days)
  • "52-week high/low" (annual range)
  • Weekly options expiration (Fridays)
  • Technical analysis: "Weekly charts"

• Billing cycles:

  • "Net 2 weeks" (payment due in 2 weeks)
  • Subscription services: "Weekly plan" option
  • Rental agreements: "$200 per week"

• Economic reports:

  • Weekly jobless claims: Key economic indicator (released Thursdays)
  • Weekly retail sales
  • Reported "week ending [date]"

5. Media and Entertainment

Content published on weekly cycles:

• Television:

  • New episodes weekly (e.g., "Every Thursday at 8 PM")
  • "Weekly series" vs. "binge-release" (all at once)
  • Broadcast week: Monday-Sunday
  • Nielsen ratings: Weekly viewership

• Podcasts:

  • "Weekly podcast" (new episode every week)
  • "Bi-weekly" (every two weeks)
  • Consistent day (e.g., "New episodes every Monday")

• Print media:

  • Weekly magazines: Time, The Economist, New Yorker
  • Weekly newspapers: Alternative weeklies
  • Sunday newspapers: Largest edition of week

• Streaming:

  • "Watch time per week"
  • Binge-watching: 20-30 hours per week (heavy users)

• Social media:

  • "Posts per week" (engagement metrics)
  • "Weekly active users" (WAU)
  • "This week's trending topics"

• Gaming:

  • "Weekly challenges" (Fortnite, Destiny)
  • "Play time per week"
  • Weekly leaderboard resets

6. Religion and Spirituality

Religious observances follow weekly cycles:

• Judaism:

  • Sabbath (Shabbat): Friday sunset to Saturday sunset
  • Torah portion of the week: 54 portions cover annual cycle
  • No work, cooking, electronics on Sabbath
  • Synagogue services Friday evening and Saturday morning

• Christianity:

  • Sunday worship: Weekly church services
  • Protestant: Sunday morning service
  • Catholic: Saturday evening or Sunday Mass
  • "The Lord's Day"

• Islam:

  • Jumu'ah (Friday prayer): Mandatory congregational prayer for men
  • Friday midday, not full day of rest
  • Work permitted before/after prayer

• Other traditions:

  • Buddhism: Some temples have weekly meditation groups
  • Hinduism: Different days for different deities
  • Modern spirituality: "Weekly yoga class," "Sunday meditation"

7. Personal Planning and Habits

Individuals organize life by weeks:

• Planning horizon:

  • Most people plan "this week" and "next week"
  • Weekly to-do lists
  • "What's happening this week?"
  • Sunday evening: Plan upcoming week

• Grocery shopping:

  • Average: Once per week (Sunday or weekend)
  • "Weekly groceries" budget
  • Meal planning for the week

• Household chores:

  • Laundry: Weekly cycle
  • Cleaning: Weekly schedule (bathrooms, vacuuming)
  • Trash/recycling: Weekly pickup

• Social calendar:

  • "Let's meet next week"
  • "Busy this week"
  • Weekly dinner with friends
  • "Weekend plans"

• Habit tracking:

  • "Workout 3 times per week"
  • "Read 2 hours per week"
  • "Cook at home 5 times per week"

Conversion Guide

Basic Week Conversions

Weeks to days:

• Formula: days = weeks × 7
• Example: 3 weeks = 3 × 7 = 21 days

Days to weeks:

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

Weeks to hours:

• Formula: hours = weeks × 168
• Example: 1 week = 1 × 168 = 168 hours

Hours to weeks:

• Formula: weeks = hours ÷ 168
• Example: 336 hours = 336 ÷ 168 = 2 weeks

Weeks to minutes:

• Formula: minutes = weeks × 10,080
• Example: 1 week = 1 × 10,080 = 10,080 minutes

Weeks to months (approximate):

• Formula: months ≈ weeks ÷ 4.35
• Example: 8 weeks ≈ 8 ÷ 4.35 ≈ 1.8 months

Weeks to years (approximate):

• Formula: years ≈ weeks ÷ 52.14
• Example: 52 weeks ≈ 52 ÷ 52.14 ≈ 1 year

Comprehensive Conversion Table

Weeks	Days	Hours	Minutes	Months (approx)	Years
1	7	168	10,080	0.23	0.019
2	14	336	20,160	0.46	0.038
4	28	672	40,320	0.92	0.077
8	56	1,344	80,640	1.84	0.154
12	84	2,016	120,960	2.76	0.230
13	91	2,184	131,040	3.0	0.249
26	182	4,368	262,080	6.0	0.499
52	364	8,736	524,160	12.0	1.0
104	728	17,472	1,048,320	24.0	2.0

Note: Month conversions are approximate because months vary in length (28-31 days).

Common Time Period Conversions

Period	Weeks	Days	Notes
Fortnight	2	14	British term for "two weeks"
Three weeks	3	21	Common vacation length
Month (avg)	4.35	30	Approximate
6 weeks	6	42	Maternity leave (some countries)
Quarter	13	91	Business quarter (~3 months)
Semester	15-16	105-112	Academic term
6 months	26	182	Half year
Year	52.14	365	Common year
Leap year	52.29	366	Every 4 years (mostly)
Pregnancy	40	280	Full term

Work-Related Conversions

Work Period	Weeks	Work Days	Work Hours (40hr/wk)
1 work week	1	5	40
2 weeks (bi-weekly)	2	10	80
1 month	4.35	21-22	174
Quarter	13	65	520
6 months	26	130	1,040
1 year	52	260	2,080

Note: Work days exclude weekends; calculations assume 5-day work week.

Age in Weeks

Age	Weeks	Notes
Newborn (1 month)	4	Infant age often counted in weeks
6 months	26	Baby milestones
1 year	52	1st birthday
5 years	261	Kindergarten age
18 years	938	Legal adult (US)
30 years	1,564	Young adult milestone
50 years	2,607	Mid-life
75 years	3,911	Senior years
100 years	5,214	Century mark

Common Conversion Mistakes

1. Assuming 4 Weeks = 1 Month

WRONG: "4 weeks = 1 month exactly" CORRECT: 4 weeks = 28 days, most months have 30-31 days

Why this matters:

• 4 weeks = 28 days
• Most months = 30-31 days
• Average month ≈ 4.35 weeks (30.44 days ÷ 7)

Examples:

• "Monthly payment" ≠ 4 weekly payments
• Pregnancy: 40 weeks = 280 days ≠ 10 months (actually 9.2 months)
• February: 4 weeks exactly (non-leap), but other months longer

Correct calculation:

• 1 month (30 days) = 30 ÷ 7 = 4.29 weeks
• 1 year = 52.14 weeks, not 48 weeks (12 × 4)

2. Work Weeks vs. Calendar Weeks

WRONG: "Working 2 weeks = 14 days of work" CORRECT: Working 2 weeks = 10 work days (Monday-Friday only)

Confusion:

• Calendar week: 7 full days (Monday-Sunday)
• Work week: 5 work days (Monday-Friday), excludes weekend

Examples:

• "2-week notice" at job = 10 work days (2 weeks of work)
• "2-week vacation" = 10 work days off (14 calendar days)
• "Complete in 1 week" = 5 work days or 7 calendar days? (clarify!)

Project planning:

• Software sprint: "2-week sprint" = 10 work days
• Contractors: Paid for work weeks, not calendar weeks

3. Bi-weekly vs. Semi-monthly

WRONG: "Bi-weekly = twice per month" CORRECT: Bi-weekly = every 2 weeks (26 times per year)

Definitions:

• Bi-weekly: Every 2 weeks = 26 pay periods per year (52 weeks ÷ 2)
• Semi-monthly: Twice per month = 24 pay periods per year (12 months × 2)

Financial impact:

• Bi-weekly: Two "extra" paychecks per year (2 months with 3 paychecks)
• Semi-monthly: Consistent 2 paychecks every month
• Budgeting differs significantly

Which months have 3 bi-weekly paychecks?

• Depends on start date
• Typically 2 months per year

4. Pregnancy "9 Months" vs. 40 Weeks

WRONG: "40 weeks = 10 months" CORRECT: 40 weeks ≈ 9.2 months

Calculation:

• Pregnancy: 40 weeks standard
• 40 weeks = 280 days
• 280 days ÷ 30.44 (avg days/month) = 9.2 months
• Not 10 months!

Why confusion?

• People think 4 weeks = 1 month
• 40 ÷ 4 = 10 months (wrong!)
• Actually: 40 ÷ 4.35 ≈ 9.2 months (correct)

5. 52 Weeks vs. 365 Days

WRONG: "52 weeks = 365 days exactly" CORRECT: 52 weeks = 364 days (1 day short of year)

Accurate calculation:

• 52 weeks × 7 days = 364 days
• 1 year = 365 days (366 leap year)
• 1 year = 52.14 weeks (52 weeks + 1 day)

Implications:

• Annual events drift by 1 day (2 days in leap years)
• "Same week next year" is actually 1-2 days later
• Calendar dates don't align with week numbers year to year

6. Week Numbers and ISO 8601 Confusion

WRONG: "January 1 is always Week 1" CORRECT: Week 1 is the week containing first Thursday of year

ISO 8601 standard:

• Week 1 = first week with 4+ days in new year
• Equivalently: Week containing first Thursday
• Result: Week 1 can start in December!

Example:

• January 1, 2024 (Monday) = Week 1, 2024
• January 1, 2023 (Sunday) = Week 52, 2022! (because it's the last week with 4 days in 2022)

Why this matters:

• Business reports: "Week 1 sales"
• International standards
• Software date handling