NOTE: The cauldron's contents are determined based on the date when you first load its area each game, and again each NG+ cycle.
The date it uses is based on UTC time, which is probably different from your local time.
WARNING: In December of leap years, contents will be selected at random due to what is likely a bug.
Contents will still be either Void Liquid or Air but will differ unpredictably, even at the same time of day and on the same seed.