If you need to group times into buckets (i.e. group by 6 hours, group by 3 hours, etc.) you can do so with a rounding function called FLOOR.
In the example shown, we have a number of transactions, each with a timestamp. Let's say you want to group these transactions into buckets of 3 hours like this:
12:00 AM-3:00 AM
3:00 AM-6:00 AM
6:00 AM-9:00 AM
9:00 AM-12:00 PM
For example, a time of 2:30 AM, needs to go into the 12:00 AM - 3:00 AM bucket. A time of 8:45 AM needs to go into the 6:00 AM-9:00 AM bucket, and so on.
If you think about it, one way to do this is to round each time until it fits into the right bucket. However, unlike normal rounding, where we might round to the nearest multiple, in this case, we want to round down to the nearest multiple, starting at midnight.
Because Excel times are just decimal numbers, you can easily do this with the FLOOR function, which rounds down to a multiple that you supply (FLOOR calls the argument that represents multiple "significance"). Even better, FLOOR understands how to round time provided in a format like "h:mm" (for example, "3:00", "12:00", etc.).
In the example shown, the formula in E5 is:
FLOOR knows how to read time, so it interprets 3:00 as its decimal equivalent, 0.125. It then simple rounds down each time to the nearest multiple of 0.125 You can use this same approach to group times into any standard bucket that you like.
If you have times that span one or more days, you can use the MOD function to extract just the time, as explained here.
Pivot tables will automatically group times into buckets of 1 hour, but they can't automatically group into other time buckets. However, using the approach outlined on this page, you can group time as you like, then run the resulting data through a pivot table to summarize.