This program (source here) adds multiples of the mean tropical year (n*365.242189*86400) to your birthday with the addSeconds() function of the php pear Date package. It does not consider time zones so if you enter local time it should print local time correctly - I hope. You probably get 1h wrong results if your birthday jumps between daylight savings time and standard time. Leap seconds are enabled, but they are, of course, unknown for the future. Leap seconds will only be added at the correct moment if you use UTC but they don't matter in most cases, anyways.
Privacy: Your date and time of birth will be transmitted with the POST method and so will not be stored in the web server log files unless you use URLs such as http://bksys.at/bernhard/birthday.php?date=19700101123000 .
Please send any bugs, comments, etc. to me (email address protected by intelligence test).