localtime giving different results for tm_hour/tm_isdst on Android and Windows

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I've noticed that localtime seems to give different results on different platforms, despite the fact that every definition I can find seems to define it the same way.

I run the following code on Windows:

struct tm *tm1;
time_t ltime = (time_t)0;
tm1=localtime(&ltime);

then edit, recompile and run the following:

struct tm *tm1;
time_t ltime = (time_t)60*60*24*30*6;
tm1=localtime(&ltime);

The results are the same except for a difference of roughly 6 months and more importantly the following:

tm1.tm_hour = 00000000 //epoch
tm1.tm_hour = 00000001 //epoch + 6 months

This is expected since epoch (Jan 1st 1970) is not in daylight savings time for my timezone (GMT) but it has entered DST after 6 months. This demonstrates that Windows correctly applies DST rules.

However, when I do the same on Android I get:

tm1.tm_hour = 00000001 //epoch
tm1.tm_hour = 00000001 //epoch + 6 months

I have no idea why the results aren't consistent, this is surely about as simple as it gets! The only thing I can imagine is that, rather than applying DST rules, it is getting the localtime at epoch=0 for my current timezone, BST(GMT+1). Incidentally I've also done this on iOS, OSX and Linux, which also display the same problem as Android.

The clearest definition of localtime I can find seems to suggest that it should work as it does on Windows:

If an application uses localtime() to determine local time, localtime() will determine if daylight savings applies (assuming DST info is available) and will correctly set the tm_isdst flag.

tm_isdst is also stuck at 0 on the faulty platforms but changes on Windows, as it should.

Some obvious checks out the way:

• Timezone: All devices are currently set to the same timezone


• Hardware clocks: they seem synced but it shouldn't matter as I am passing in my own time value anyway, zero. localtime only requires the time value you pass in, the DST rules and the timezone according to TZ.

asked 48 secs ago

user3640654

1