I may be a bit dim, but could someone please explain again the functionning of the Fischer clock?

Example: I have a game that started 5 days ago, with setting "Time: 7 days, Bonus: 1 day, Limit: 15 days, no days off". We are now at move 17, which means that the "time left" climbed slowly up to the 15 day limit, and that's where it is now, 5 days into the game: both players have 15 days "time left".

But is that the way it was supposed to be? if the max "time left" is 15, shouldn't it read now 15days minus the time we have been using so far?