Thank you for the opportunity. It is stated that the Jewish day starts at nightfall, and continues throughout the night and following day, until the next night. When does the date change? If it is Nissan 14th in the daytime - is the date advanced to Nissan 15th just after sunset or does that only happen after midnight?

Hello. As you said, the day starts from the night, therefore the date changes as well right after the sunset. All the best!