This yml file shows the relationship between the tz_database values for regions (e.g. Europe/London) and the Fortigate "system global timezone" value (e.g. 25 = Europe/London).
The TZ Database names are taken from a Wikipedia entry on TZ values, and the Fortigate values are found by me splitting the strings (on comma's) from when I have tried auto-completing the timezone field on a Fortigate model.
It is not a perfect list! Updates welcome!!