There is a daily limit of 200 rep for things like upvotes on answers. What constitutes a day though? Is it a rolling 24 hour period? 12:00am to 12:00am GMT? Something similar with my own time zone?
Depends on the particular limit.
Rate-limiting, such as the limit on the number of questions asked, is based on a rolling period. That is, the daily limit for questions is 6 questions in any 24 hour period. See Time reset/ending time for "Sorry, you are only allowed to ask 6 questions in a 24 hour period" For more information in rate-limiting, see The Complete Rate-Limiting Guide. Did you know that you can change your Winterbash hat only once every 10 seconds?