减去1ms会导致意外行为

Question

What is wrong? I assume that if I subtract 1ms from 1 Jan 1980 0:0:0 then I've got 1979. But I must subtract about 500+ ms for this. Please, give me a hint.

val cal = Calendar.getInstance(TimeZone.getTimeZone("UTC"))
cal.set(1980, 0, 1, 0, 0, 0)
val date = new Date
date.setTime(cal.getTimeInMillis()) // <- 1980 Jan 01 0:0:0
date.setTime(cal.getTimeInMillis() - 1) // <- 1980 Jan 01 0:0:0 too !!!

Updated.

The solution is

val cal = Calendar.getInstance(TimeZone.getTimeZone("UTC"))
cal.setTimeInMillis(0)
cal.set(1980, 0, 1, 0, 0, 0)

Answer 1

With Calendar.set(year, month, day, hourOfDay, minute, second) no milliseconds are set. Consequently the Calendar implementation sets the milliseconds to "unknown" which is actually treated as the midpoint within the given second.

Subtracting 500ms means you just step over the midpoint. Same should happen if you add 500ms, which should bring you just over the second. Actually subtracting 500ms works and you must add 620ms to see the next second.