The countdown is very finicky in my opinion. Have you checked out the user manual on this subject? I know it's been documented there pretty good.
Word of caution... the countdown generated by xLights is not dynamic. Once you generate the fseq file, the countdown is locked in stone and will countdown the same each and every time you run the sequence.
If a dynamic countdown is needed, then you would need to use FPP or xSchedule.
