"minuteswest" element returned by gettimeofday()... concerning Daylight Savings Time

MarkB423

My question deals with the gettimeofday() function and one of the array elements it returns called "minuteswest" (which, I should note, actually appears to be seconds west... anyone else notice this?)...

The "minuteswest" element is the timezone offset (which seems to be returned in seconds) that I use to calculate the difference between the server's timezone and the timezone that the user of my program wants to have displayed (probably 99.99% of my clients will be in the U.S.).

The question is... say the server is in the Eastern Timezone (GMT -05:00)... right now the "minuteswest" element returns 18000 (300 mins), BUT in the middle of summer during Daylight Savings Time, will this value now be 21600 (360 mins)? Or does DST not affect the timezone offset returned by this PHP function?

I know that the timezone abbreviation will (or should) change from EST (eastern standard time) to EDT (eastern daylight time), but does the timezone offset change from -05:00 to -06:00?

Weedpacket

As noted, this is an interface to the library function gettimeofday(). As such, it depends mainly on the C library used in building PHP.

My own library's man page on the subject says regarding gettimeofday(2):

struct timezone {
               int  tz_minuteswest; /* minutes W of Greenwich */
               int  tz_dsttime;     /* type of dst correction */
       };
The use of the timezone struct is obsolete; the tz_dsttime field has never been used under Linux - it has not been and will not be supported by libc or glibc. Each and every occurrence of this field in the kernel source (other than the declaration) is a bug. Thus, the following is purely of historic interest.

The reason it was abandoned was that

it turned out that the period in which Daylight Saving Time is in force cannot be given by a simple algorithm, one per country; indeed, this period is determined by unpredictable political decisions.

I recommend you use the time() and date() functions.

MarkB423

Thanks for notifying me of that. BUT...

The main question is whether or not timezone offset is changed with the changing from Standard Time to Daylight Savings Time. There are other places to get the offset of the current server time than the gettimeofday() function (that I will no longer use thanks to your input)... I believe the date() function can return the offset, in the form of -0500, for instance, for the Eastern Standard Timezone.

Now, during Daylight Savings Time, will this change to -0600 or remain at -0500, and just have an hour added to it?

My main worry is referencing old dates based on their Unix Timestamp and getting the correct hour (based on whether or not it was DST or not). BUT I am also trying to allow the user to decide the offset that should be displayed, but I am not sure if timezone offsets change with DST...

Weedpacket

They should do - it depends on how your system is configured (its a localisation issue). If you can, you could experiment by deliberately changing your system's clock temporarily.