You are correct in pointing out the discrepancy in the opening times - when the Bargello is hosting a special exhibition, the hours are extended to 6 p.m. but generally the official opening times are from 8.15 a.m. to 1.50 p.m., with the ticket office closing at 1.20 p.m.
The Bargello's official page here
Polo Museale Fiorentino has information on the special exhibitions it has planned. Currently there is one called "Great Bronzes of the Baptistery" at the Bargello. Not only are the hours different (extended), the entrance fee is also higher during these special exhibits. This is also the case for the Uffizi and Accademia Gallery when they have special exhibitions.
Also make special note that Mondays are particular days for museums, many are open on certain Mondays of the month and closed on others. For example, the Bargello currently says that it is open on the 1st, 3rd and 5th Monday of each month (so it is closed on the 4th Monday of the month).
I know this can be quite confusing so please check that these hours won't affect your plans for your one day in Florence! Most other museums also have these variations in opening hours, so keep that in mind when planning your visit.