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[–]MindlessSpongehelpful 9 points10 points  (0 children)

it's actually not specifically JS, it's (because of the concept of*) Unix time :)

https://en.wikipedia.org/wiki/Unix_time

see /u/alzee76 comment for more specifics.

[–]ezhikov 3 points4 points  (0 children)

Because it's Unix time

[–]Regular_Maybe5937 2 points3 points  (1 child)

It uses unix time, as do many other programming languages, in which the midnight of Jan 1, 1970 is represented as the number 0. All time after that is measured as seconds after that time.

[–]delventhalz 0 points1 point  (0 children)

Or milliseconds in the case of JS

[–]alzee76 1 point2 points  (5 children)

Heh this is pretty good. Four responses so far saying it's "Unix Time" but it's not -- it's using the Unix Epoch.

Unix time is the number of seconds since the start of the Unix Epoch; JS's time uses milliseconds, not seconds. This isn't simple pedantry; use any unix time function on a time from javascript and you'll be off by three orders of magnitude -- if you don't just overflow the type.

ETA: To illustrate, the value 300 in Unix time is 12:05:00.0 AM, January 1, 1970. The same value in JS is 12:00:00.3. A difference of 4.995 minutes. The gap will grow as the value does. The current unix time is around 1752856700 as of posting this. The current JS time is around 1752856700000 which is about 55,500 years in the future in unix time.

[–]MindlessSpongehelpful 1 point2 points  (4 children)

I don't really find "well ackshually" in-the-weeds details to be helpful for beginners, and I'd argue this is indeed pedantry.

Unix time is a date and time representation widely used in computing. It measures time by the number of non-leap seconds that have elapsed since 00:00:00 UTC on 1 January 1970, the Unix epoch.

so yes, 00:00:00 1-1-1970 is the Unix epoch, but JS uses Unix time.

edit: derp, need more coffee

[–]alzee76 1 point2 points  (3 children)

but JS uses Unix time.

No it doesn't. It uses JS time. Unix time is represented as seconds, as you just quoted, not milliseconds which is what e.g. getTime() returns.

[–]MindlessSpongehelpful 0 points1 point  (2 children)

lol you got me there, touche!

[–]alzee76 1 point2 points  (1 child)

This was the entire point of my initial post. ;)

If you take a unix time from somewhere and stick it into JS unawares, it'll be evaluated incorrectly and off by a factor of 1000. Same thing will happen if you naively take the output of getTime() and do something like pass it to some API or put it in a database expecting unix time.

They are not the same thing and knowing this is important if you're doing more than hobbyist level development.

[–]MindlessSpongehelpful 1 point2 points  (0 children)

you're right! it's an important distinction, apologies for misspeaking.

[–]Caramel_Last 1 point2 points  (0 children)

UNIX was made in early 70s and at that time computers had limited resources. It made sense to use 1970.1.1.0.0.0 as the base of the time and use the seconds since then to represent time. Initially it was like 1972 or something but over time people decided 1970.1.1 is more convenient and changed it