Use boost date_time.
It's one of the boost bits that comes in a library rather than just a header file but don't let that put you off, it makes easy some things that were hard, and makes possible some things that were impossible. It is a good thing.
The rest of these rules will focus on the more prevalent posix types and functions.
Never put anything other than posix time in a time_t.
By posix time I mean time since midnight 1st Jan 1970 UTC, aka epoch time, or unix time, or timestamp. You will have a hard time persuading anything to treat your time_t value as anything else so just don't do it.
Never put anything other than the local time in a struct tm.
Effectively this means never calling gmtime. There is only one reason to ever call gmtime, and that is if you need to get a UTC time string, in which case don't keep the value around for any longer than it takes to create the string. Doing so is asking for trouble.
Anybody writing code that relies on being run in a particular timezone shall be taken outside and shot.
This should be obvious and strikes me as lenient, personally I would beat them with sticks until dead. Shooting's too good for them.
Anybody writing tests that rely on being run in a particular timezone shall be taken outside and beaten with sticks until they are sorry.
It may look like an easy and harmless shortcut but No, just No!
Never go near the TZ environment variable.
Oh TZ environment variable, how I hate thee? Let me count the ways:
- You are not thread safe
- You can be changed invisibly by other parts of the codebase.
- You are hard to clean up correctly.
- You do not even bloody work properly.
Yes, it turns out that Windows is, as you'd expect, expertly engineered to work in every location you can imagine except if you change the TZ environment variable, after which it will get the timezones right (but of course), but likely get the dates of change from standard to daylight saving and back again wrong. it will use the American dates, even when it is not in America.
Alter your timezone and check it again.
I have come to the belief that writing time-handling code in a timezone that coincides with UTC for half the year is an actual disadvantage allowing bugs to go unspotted. Now, whenever I go near it, I tell my dev box it is in Road Town.
The weather's better too.
