Is it possible to change the iOS Simulator to use 24 hour time?

Time zone can be changed by system only System Preference/Date & Time select time zone and change.After changing time zone by system it will reflect in your simulator. iPhone simulator has no option to change time zone.

If you switch the region format to a country that uses 24-hour time as a standard (e.g. France) inside the simulator then it should work for you.

Go to Simulator / Settings / General / International / Region Format / Select the Region