I have 5 times in a day that an event happens :- 02:24 04:24 08:24 12:24 16:24

I have a cell(a) that I put a time into this could be anytime from the last 48 hours

How do I display in a cell(b) the NEXT event time based on the time I put into the cell(a)

So if I put it 05:47 in cell(a)
Cell(b) would display '08:24'

Thanks in advance