Second, minute, hour, day — clean conversions and 1 tricky case
Everything in the time category converts trivially except month-year boundaries.
Within the SI time category, every conversion is exact:
- 1 s = 1000 ms = 1,000,000 μs = 1,000,000,000 ns - 1 min = 60 s - 1 h = 3600 s - 1 d = 86,400 s - 1 wk = 604,800 s - 1 fortnight = 1,209,600 s (= 2 weeks)
The two units where the conversion is not exact:
- Month: 28–31 days depending on the calendar month and year. - Year: 365 (non-leap), 366 (leap), 365.25 (Julian), 365.2422 (tropical), 365.2425 (Gregorian average).
This site picks the Julian year (365.25 days, 31,557,600 s) for consistency. Calendar months and years are not in our table because they aren't well-defined units — they're calendar periods.
For most practical use this is irrelevant. The interesting exceptions are: leap-year handling in billing (some subscription services charge 1/365 vs 1/366 depending on date), broadcast media scheduling, and pregnancy/maternity arithmetic (40 weeks ≈ 9 calendar months ± a few days).