Births and deaths are created within several days of the event. The first date that you will find in the birth record or death record is the record date. Further down in the record, you will find the actual birth or death date, which could have been up to five days earlier.
Indexes sometimes only have the record date, so make sure to always check the original record.
Marriages were recorded on the day of the marriage, so there is no difference between the date of the event and the record there.