Time units explained
Everything up to the week is exact and has been for millennia. Months and years are where it falls apart: they are astronomical periods that refuse to divide evenly, so any single number for them is an average rather than a fact.
Nanosecond ns
1 ns = 1e-9 s Exact
A billionth of a second. Light travels about 30 cm in one, which is why nanoseconds are a practical limit on how fast distant parts of a computer can talk to each other. Signal timing on a circuit board is measured here.
Microsecond µs
1 µs = 0.000001 s Exact
A millionth of a second. Camera flash durations, the timing of individual operations inside older processors, and the resolution most programming languages give you when you ask what time it is.
Millisecond ms
1 ms = 0.001 s Exact
A thousandth of a second. Network latency, screen refresh and audio delay all live here — the human eye stops noticing individual frames somewhere around 40 ms, and input lag becomes perceptible at about 20.
Second s
1 s = 1 s Exact
The SI base unit, and no longer defined by the Earth at all. Since 1967 it has been 9,192,631,770 oscillations of a caesium atom, because atomic behaviour is far more reliable than the planet's rotation, which quietly varies.
Minute min
1 min = 60 s Exact
Sixty seconds, inherited from Babylonian arithmetic in base sixty. That system survives in exactly two places in modern life: clocks and angles, both of which divide by 60 twice.
Hour h
1 h = 3,600 s Exact
Sixty minutes. The twenty-four hour day comes from Egypt, where day and night were each divided into twelve — which meant hours changed length with the seasons until mechanical clocks made that untenable.
Day d
1 d = 86,400 s Exact
Fixed at exactly 86,400 seconds as a unit, even though the Earth's actual rotation drifts. Leap seconds have occasionally been inserted into civil time to reconcile the two, letting the unit stay constant while the calendar absorbs the difference.
Week wk
1 wk = 604,800 s Exact
Seven days, and the only common time unit with no astronomical basis whatsoever. It does not divide the month or the year evenly, which is why a date falls on a different weekday each year, and it has survived several deliberate attempts to replace it.
Month mo
1 mo = 2,629,746 s
Somewhere between 28 and 31 days depending on which one you mean, so the figure used here is the Gregorian average of 30.44 days. That is a convention, not a fact, which is why month conversions carry no exactness badge — if you need a real interval, count actual dates instead.
Year yr
1 yr = 31,556,952 s
365.2425 days on the Gregorian average — the fractional part is why leap years exist, and why century years are skipped unless divisible by 400. A common year is 365 days and a leap year 366; neither is the number used for conversion here, because neither holds over the long run.