Why do timestamps have a limit to 2038?

Question

I just found out, running a calendar script, that timestamps in PHP has a limit to 2038. What does it really mean? Why is it 2038 instead of 2050 or 2039? Why a limit if timestamps just count seconds from a given date (1970)?

Carve

#5320210 – Morel

en.wikipedia.org/wiki/Y2K38 – Singapore

@muistooshort Hahaha, that was my thought exactly, as soon as I read this. "Uh oh. We're all going to die." – Pushover

Answer 1

The limit is imposed by the 4 byte signed integers that most C libraries use for representing that count. Quick math (assumes 365 day years, not exactly correct):

2147483648 seconds ~ 68.1 years

This also implies a lower limit of ~1900. Some libraries have started to introduce 64 bit epoch counts, but they are few and far between for the moment.

Hiett

Why cannot they add another int and make the sum when the limit is reached? – Carve

@Charliepiga check out my updated answer. It's a work in progress, but given that it won't be an issue for a while for most people, it's slow going. – Hiett

Yes, but why limit at all? Couldn't they just add something like if (timestamp > limit) { add another int and start a new count from there, adding int1 + int2 to get the total } ? That would let us other 68 years more... – Carve

@Charliepiga PHP is in some ways limited by the C libraries it relies on. Time in particular is a very tricky topic when you start trying to convert a UNIX timestamp to an actual human readable date/time. Go have a look at the mailing list for any date/time library. I know I just gave up after listening in on the nodatime mailing list for a while. – Hiett

@Charliepiga also, all values have to have a limit somewhere. There's no way to store an infinite data in a finite space (sadly!). Using 64b integers for the time count you can represent huge timespans, but as I said, the support on the C side is lacking at the moment. If you're lucky enough to have a PHP compiled against one of these C standard libraries you shouldn't actually have this limit if I'm reading between the lines in the documentation correctly. – Hiett

Answer 2

The maximum value of a signed 32-bit integer is 2,147,483,647. If you add +1 to that, you get -2,147,483,647. 2,147,483,647 seconds from 01-01-1970 00:00:00 is January 19, 2038. If you add one more second, you get a date somewhere in 1902.

Rossner

Answer 3

due to the limit of INT datatype on 32 bit machine

http://php.net/manual/en/function.mktime.php

From php.net : "The maximum possible date accepted by mktime() and gmmktime() is dependent on the current location time zone.

For example, the 32-bit timestamp overflow occurs at 2038-01-19T03:14:08+0000Z. But if you're in a UTC -0500 time zone (such as EST in North America), the maximum accepted time before overflow (for older PHP versions on Windows) is 2038-01-18T22:14:07-0500Z, regardless of whether you're passing it to mktime() or gmmktime()."

Embrangle

Answer 4

my guess is that it is stored in a fixed number of bits, which means a limit on how big the timestamp can get. We could do some math to figure it out exactly.

Ryan