As others have said, the marriage was unlikely to have taken place in the church itself. This only became common in Scotland in the 20th century.
As to the date given in marriage registers, it varies considerably. Usually it is the date of the "contract" or "booking", when the couple went to the session clerk and said they intended to marry. They were supposed to marry within forty days of that. Sometimes a record also includes the dates on which the banns were read. Sometimes the actual date of the wedding itself is recorded. You can find any one of these, or a combination.
If a single date is given and there is no statement anywhere on the page or in the entries that the date is that of the marriage, best to assume it is the date of the initial contract.
