Comparing time_t values using comparison operators

Question

I have 2 time_t values and I want to find out which one is greater.

time_t is internally __int64 on my platform. Can I use <, > and == operators to compare the values? I don't want to know the difference between the two time values.

The code will just run on Windows, so I don't care about portability.

Is it correct to compare the values in this way?

If you don't care about portability and know that `time_t` is internally `__int64`, then what is the point of the question? `__int64` values are certainly comparable with comparison operators. — user4815162342, Aug 08 '14 at 09:07
Did you google `time_t` before asking this question? I did. [This](http://www.cplusplus.com/reference/ctime/time_t/) was the first result. It told me all I needed to know. Please do research before asking questions on SO. — Michael Dorst, Aug 10 '14 at 09:30
@anthropomorphic i had already seen that page, but i purposely wanted to ask this, because, that page does not explain as much as the techies below.. — 51k, Aug 11 '14 at 06:29

filmor · Accepted Answer · 2014-08-12T07:15:26.313

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According to section 7.27.1(3) of the C standard (which the C++ standard refers to in this case) time_t is a real type, which is defined in 6.2.5(17) as either integer or floating type.

As long as you don't explicitly assume integers (i.e. by doing time_t t; int x = t;) which could lose precision you are safe.

EDIT: I take that back actually. If you go to this link, post #6, you can read the following:

Just because time_t is arithmetic, that doesn't mean it stores time as monotone increasing values for advancing time. In fact, our Standard C library does not guarantee t1 < t2 for t2 later than t1. We fold a limited range of representable values different ways for different systems.

P.J. Plauger
Dinkumware, Ltd.
http://www.dinkumware.com

Since Dinkumware is widely used on Windows systems (although I don't know whether this also holds for the C++ standard library) I consider this relevant.

tl;dr: Use std::difftime as suggested in the other answers.

edited Aug 12 '14 at 07:15

answered Aug 08 '14 at 07:42

filmor

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There is probably quite a bit of code that would break if time_t suddenly became a struct or array... – Mats Petersson Aug 08 '14 at 07:43
Yep, looked it up. According to the standard it has to be a real numeric type, so you're right. – filmor Aug 08 '14 at 08:26
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`t0 < t1` is safe, but the standard doesn't guarantee that is meaningful. See my answer. – Keith Thompson Aug 10 '14 at 10:35

Keith Thompson · Answer 2 · 2019-03-18T19:30:11.107

The C standard (from which C++ inherits the definition of time_t) says only that time_t is an arithmetic type capable of representing times. It says nothing about how it does so. In principle, an implementation could define a time_t value as the number of seconds until some future date, or it could encode month, day, year, hours, minutes, and seconds in that order. Comparing time_t values is guaranteed to behave consistently (t0 < t1 && t1 < t2 implies t0 < t2, for example), but the result of a comparison doesn't necessarily say anything about the order of the times that are represented.

To be 100% portable, you can check the result of difftime().

But if you're willing to settle for about 99% portability, you should be safe in assuming that comparisons work as expected (that t0 < t1 implies t0 precedes t1 in real time). I've never heard of an implementation where that doesn't work. If it falls, you can complain to the implementers, but you can't say they've failed to confirm to the standard.

score 0 · Answer 3 · answered Aug 08 '14 at 07:42

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Yes, you can use normal comparative operators in the limited case you give. There may be other systems where this doesn't work, but modern Linux and Windows OS's will work fine to compare time_t values.

answered Aug 08 '14 at 07:42

Mats Petersson

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According to the standard as quoted by filmor's edited answer (ironically prompted by your comment), compating `time_t` values should always work. That makes the claim in this answer that "there may be other systems where this doesn't work" incorrect. – user4815162342 Aug 08 '14 at 09:16
@user4815162342: Depends on what you mean by "work"; see my answer. – Keith Thompson Aug 10 '14 at 10:36
@KeithThompson Well spotted, +1. It is strange and surprising that the standard allows an implementation to use `time_t` as something other than count of time units since the epoch, but the wording is clear in 7.23.2.4#2 of C99 and in 4.2.12.4 of C89: *The encoding of the value is unspecified.* – user4815162342 Aug 10 '14 at 18:01

score 0 · Answer 4 · answered Aug 08 '14 at 08:53

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Assuming time_t has a high enough resolution for your use case then yes you can. If you need a higher resolution and Windows is your only concern then consider switching to the Win32 functions that deal in the FILETIME type.

answered Aug 08 '14 at 08:53

Andy Brown

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Comparing time_t values using comparison operators

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