Unless I'm being silly, shouldn't the % just be / here?

We still want it to wrap around 60 as that's our current unit of time. FormatTime is likely the most correct fix here.

FormatTime is not appropriate and a time stamp must be used

Some solutions:

1. limit sm_vote_delay to 3600 seconds to cover the only corner case
2. or move function int FormatTimeDuration(char[] buffer, int maxlen, int time) from nextmap.sp to another place (maybe in timers.inc as stock) and use FormatTimeDuration to format the delay on basevotes.sp and funvotes.sp

Unless I'm being silly, shouldn't the % just be / here?

Ok corrected as your suggest