setting daylight savings time in a php application

I have a developer just finishing an application for me in php that sends users messages at specific times during the day.

We've become stumped as to how to handle daylight savings time (summertime in europe) changes.  My server automatically adjusts for DST and thus so does my applications perceived time.

All of the users choose their timezone but the application doesn't make any adjustments for DST.

What is the cleanest way to address this?  Are there pre-built modules that can be used?

I'm not a programmer so please keep any responses in english... :-)

Thanks