Date/time support has been fully rewritten in PHP 5.1.x, and no longer uses the system settings to 'know' the timezone in operation. It will instead utilize, in the following order:
The timezone set using the date_default_timezone_set() function (if any)
The TZ environment variable (if non empty)
"magical" guess (if the operating system supports it)
If none of the above options succeeds, UTC
To ensure accuracy (and avoid an E_STRICT warning), you will need to define your timezone in your php.ini using the following format:
date.timezone = Europe/London
The supported timezones are listed, in this format, in the timezones appendix.
Also note that strtotime() now returns FALSE on failure, instead of -1.