Help! gmt_to_local, how to identify the daylight saving time(DST)?

[eluser]mindprojects[/eluser]
I've seen that the "gmt_to_local" function get an optional third parameter called "Daylight saving time".
The problem is : How it is possible to know if the daylight saving is set to TRUE or FALSE?
Do i need to set an option for the user into the settings panel to specify if the daylight saving is TRUE or FALSE?

Thanks in advance