Custom fill series

Hi,

A very basic question but I can't figure out how to create a custom fill series that changes every 'nth' cell. In my spreadsheet I have values for hh:mm and am trying to fill series that changes every 7th cell, as follows:

8:13
8:13
8:13
8:13
8:13
8:13
8:14

I have tried manipulating the series option in the fill tab and tried using the autofill by selecting the previous cells as a pattern. I'm not sure if the problem is that my data is in a difficult to use format or I am just not doing it correctly. Any help is appreciated!! Thanks!

Brilliant! Thank you so much!