As the show deals with time travel, it is only fitting that each episode title relates to that. In the first season, all episodes have the word "Time" in their titles, such as "Wasting Time" or "Playtime".
In the second season the writers expanded on the idea and named each episode something with "Second". This can be read as both the time measurement, but also an indication of the season to which the episodes belong.
Keeping in line with the previous seasons, all episodes in Season 3 includes the word "Minute" in their titles.
Continuing the trend set by the previous seasons, all episode titles the final season contain the word Hour. Very fittingly, the final episode of the final season is called Final Hour.
