[glopomcyte]

I don't know if I've got the correct times, but I've been fasting from 4am to 8pm. It gets dark at around sometime between 6pm and 7pm, so I've been going for 8pm just to be on the safe side

You can use Accurate Times to calculate the prayer times for your area. The time for fasting in Phnom Phen Cambodia today is from 5:31 a.m. to 7:28 p.m. (according to Hanafi madhab, taking summer time into account) but there are a number of variables to enter to get a more accurate time and the time will change slightly over the days.

5:31 a.m. is the time fajr starts so you need to stop eating/drinking etc. before this time. 7:28 p.m. is the time for maghrib and you should not delay breaking your fast (i.e. break it when you hear the adhan/the time has come).