MS Project Custom Fields Formula to calculate start and finish in an other timezone, but also taking in to consideration Day Light Saving.

[BVOPP]

In Custom Fields type Date we now use:
ProjDateAdd([Start];"8h";"24 hours")
ProjDateAdd([Finish];"8h";"24 hours")

But we need to add a condition to check if Day Light Save is Winter or Summer because then the difference is not 8 hours but f.e. 9.

How can i do that?