If I understand correctly, you are asking how you can increment the timecode by an hour while keeping the leading zero, is that right?
My first thought was that, since there is to be a leading zero for values < 10, we have to think not just about numbers but about strings of text. However, before I got deep into that, I found this in the forum:
Formatting numbers under 10 with a leading zero
The reply by @Tom contains a macro which demonstrates how to handle leading zeros using the "with format" option of the Set variable to calculation action.
That option does not appear by default in this action: it is necessary to enable "format result" in the settings for the action (accessed via the cogwheel icon).
PS: I wonder whether others agree that, if this information is in the Wiki, it is not easy to find.