Missing Time

The house clock is, presumably, a precise time keeper and can tell you how long you were out of the house, although not the real world times of departure and arrival. The town clock is both accurate and precise, able to tell you what time it was when you passed it going, what time it was when you returned, and how long you spent at the convention (or more precisely, how long occurred between town clock viewings).

Take the time that the house clock says you were gone, and subtract from it the time the town clock says you were gone, and this will give you the amount of time you spent walking to and from where you saw the town clock. Divide this number in two (you can do this because of your constant strolling speed), and when you walk in the front door, this is how much time you need to add to the town clock's time for the correct time now.