JavaScript converted FILETIME is one day off

Question

I have been reading about the JavaScript 'off-by-one' for dates, but I cannot figure out how to implement it in this situation. I'm converting some values from an Internet Explorer cookie into a Microsoft FILETIME, but it's always ahead by one day, the code is:

function ConvertToFiletime(high, low) {
    var seconds = 1e-7 * (high * Math.pow(2, 32) + low) - 11644473600;
    var date = new Date.UTC(1970,1,1);
    date.setSeconds(date.getSeconds() + seconds);
    return date;
}

This does the conversion perfectly well, but I can't work out the best way to deal with the extra day, which I believe is due to a lack of a timezone. Do I need to subtract a day's worth of seconds? Or is there a better way.

You should always use the UTC methods to avoid timezone issues, only use the non UTC methods when displaying the time — Ruan Mendes, Oct 31 '18 at 18:51
Not sure if this is your problem, but `Date.UTC(1970, 1, 1) ` is February 1st. You probably want `Date.UTC(1970, 0, 1)` for January 1st. — Clint, Oct 31 '18 at 19:19
Clint, that totally was the problem! Thanks for noticing that. — Tony, Nov 01 '18 at 07:56

score 1 · Accepted Answer · answered Nov 01 '18 at 16:01

1

Your error is in this line:

var date = new Date.UTC(1970,1,1);

Date.UTC(1970, 1, 1) is actually February 1st because month indexing starts at 0. You most likely want Date.UTC(1970, 0, 1) which is January 1st.

Also, new Date.UTC() throws an error for me on the latest version of Chrome and IE 11. To properly create a date object use:

new Date(Date.UTC(1970, 0, 1))

answered Nov 01 '18 at 16:01

Clint

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Thanks Clint, that solved that. I was so busy looking at the strange JavaScript date behaviour that I overlooked that silly bit :) – Tony Nov 01 '18 at 16:11

JavaScript converted FILETIME is one day off

1 Answers1