Here are some little tricks that might be useful to you if you're working with dates and times in PHP:

• A "Unix timestamp" or "epoch" refers to the number of seconds since 1970-01-01 00:00:00 UTC. (i.e. a point in time in one specific timezone.) A Unix timestamp is never timezone-specific; if you call time() at the same moment on computers in different time zones, you get exactly the same value back.
• mktime/gmmktime - Passed a number of arguments (day, month, year, etc.), returns a Unix timestamp. mktime interprets its arguments as representing a local time, whilst gmmktime interprets its arguments as a time in the time zone UTC. (That is, given the same input gmmktime returns the same results everywhere; mktime returns timezone-specific results.) Note that in this case, the gm- prefix affects how the input is interpreted.
• strftime/gmstrftime - Passed a Unix timestamp and format string, and returns a string. In strftime's case, the string is the time according to the current timezone, whilst gmstrftime returns the time in the UTC timezone. (Given the same input, gmstrftime returns the same time in all timezones (though not necessarily the same string--the actual string may be different if the locale is different) whilst strftime returns timezone-specific results.) Note that in this case, the gm- prefix affects the output string.
• strptime - Inverse of strftime/gmstrftime: passed a time string and a format string, and returns an array representing the local time. (This not a great return value; it would have been better to return a Unix timestamp.) The time string is interpreted as a local time; there is no gm- equivalent, or even any way to simulate one, since it ignores the value of date_default_timezone_set. Note that you can impose a timezone on the input string if it contains a timezone abbreviation or offset (e.g. Sun 26 Apr 2009 21:50:35 BST) which the format string reads with %z or %Z.
• date/gmdate - Passed a Unix timestamp and a format string, returns a string. As with strftime/gmstrftime, the gm- prefix affects whether the result is a representation of the time in the current timezone or in the UTC timezone. Note that the format string is completely different to that of strftime/gmstrftime! (One reason to use this set of functions is for the useful date format constants.) There is no inverse of this function.
• strtotime - Passed a string in "US English date format", and returns a Unix timestamp. Note that there is no way to override the US date format! strtotime ("03/04/2008") is the 4th of March 2008. Note that this function will parse relative times, like tomorrow and next monday; this is about the only reason to use it.
• How does PHP know what your current timezone is? It tries a few different places, including the TZ environment variable and the date.timezone ini option. (See date_default_timezone_get for more information.)