Time is the unit everyone uses daily and gets wrong professionally twice as often. The math is trivial; the conventions are not. This guide covers the conversions between technical time (milliseconds), human time (hours), and business time (work weeks), plus the calendar quirks that break software and budgets.
The fixed ladder
- 1 second = 1,000 ms = 10⁹ ns (programming's base unit since Unix time)
- 1 minute = 60 s, 1 hour = 3,600 s, 1 day = 86,400 s
- 1 week = 604,800 s = 168 hours, of which at most 40-60 are work
- 1 year ≈ 365.2425 days (the .2425 is why leap rules exist)
Work time vs calendar time: the project-killer
"Two weeks of work" means 80 person-hours. "Two calendar weeks" is 168 hours, in which one person works maybe 60-70 with meetings. Teams that estimate in work-weeks and report in calendar-weeks are always 40-60% late. The honest conversion chain: person-days × 8 → hours → ÷ (team size × focus factor ~0.7) → calendar duration. Never multiply 5-day weeks into 7-day calendar without the focus factor.
| Effort | 1 person, real | 2 people, real |
|---|---|---|
| 1 person-week (40 h) | 7-8 calendar days | 4-5 days (communication cost) |
| 1 person-month | 30-33 days | 17-20 days |
| 10 person-days | 2 weeks | 1.2 weeks |
Technical anchors
- Page-load budget: 1,500 ms to interactive or users leave (our LCP budget here is 1.2 s warn / 1.5 s fail)
- A 60 FPS frame: 16.67 ms; drop to 33 ms and users feel jank
- Ping: 10 ms metro fiber, 40 ms cross-continent, 300 ms geostationary satellite (why satellite calls feel awful)
- Unix timestamps: seconds since 1970-01-01; the 2038 problem hits 32-bit systems at 2,147,483,647 s
Calendar quirks that break things
- February: 28.24 days average. Monthly plans assuming 30 days drift 3.5 days/year against reality; subscriptions bill on the same date, not the same interval
- Leap seconds: 27 added since 1972; servers that assumed days are exactly 86,400 s have crashed for less
- Timezones: "1 day" spans 23 or 25 hours twice a year in DST zones. Flights quote local times; a 2h flight can land "before" it departed
- Week numbering: ISO says week 1 contains the first Thursday; the US counts Jan 1 as week 1. Same date, different week number, different contract deadlines
Human-scale anchors
- 1 million seconds = 11.6 days. 1 billion seconds = 31.7 years (people underestimate billion by 1,000x)
- A typical career, 40 years ≈ 2.1 billion seconds
- Deep work capacity: ~4 h/day = 1,000 h/year of truly productive time