Could this javascript date function fail?

Question

Keeping in mind DST, Leaps, Timezones.

Can this function ever screw things up? It must take a unix epoch (GMT) and figure out the exact calendar day of that epoch and return the epoch at 12:00AM of that day.

function convertToDayStartTime(epoch) {
  var d = new Date(0);
  d.setUTCSeconds(epoch);
  var dayStart = new Date(d.toDateString()).getTime()/1000;
  return dayStart;
}

For example:

expect(convertToDayStartTime(1378000800)).toEqual(1377993600);

EDIT:

Looks like it is a bit screwed up. Following test fails:

expect(convertToDayStartTime(1377993599)).toEqual(1377907200);

I am not sure how to fix.

Is it 12:00 AM local time or GMT?

Barmar
– Barmar

2014-08-25 21:11:33 +00:00
Commented Aug 25, 2014 at 21:11 — Barmar
– Barmar, Commented Aug 25, 2014 at 21:11

Patrick Collins · Accepted Answer · 2014-08-25 21:12:03Z

3

It looks to me like it will be exactly as correct as your browser's implementation of Date. However, I think you can simplify it: you know how long a day is -- 86400 seconds. So just find the nearest multiple of 86400 and go with that. You could implement it like this:

function convertToDayStartTime(epoch) {
    return Math.floor(epoch / 86400) * 86400;
}

Which gives 1377907200 for your example, as expected.

answered Aug 25, 2014 at 21:12

Patrick Collins

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2 Comments

JayPrime2012 Over a year ago

I guess that probably would be the best... i was thinking in terms of DST, etc... but I guess even with all the DST and leaping in the world, it couldn't possibly result in being over 86400/2 off the epoch time. So your solution is probably the best... (By the way I am in node not browser)

ItalyPaleAle Over a year ago

Leap seconds are not a problem, because the UNIX timestamp doesn't consider that. However, you must not assume all days last 86400 seconds...

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