In generell the dropdown filter would start at 0:00 and end at 23:00 depending on a table with timestamps. In this case the occurances of events for different hours (e.g. 05:35, 05:12 ..) from timestamp table have to be agglomerated and each hours should be distinct (e.g. 05:35, 05:12 --> 05:00).

Thank you for your help!