DB2 SQL group row count by time intervals of 10 minutes

I have a table in DB2 which contains data such as the following:

completed_timestamp details
2021-12-19-15.38.10 abcd
2021-12-19-15.39.10 efgh
2021-12-19-15.48.10 ijkl
2021-12-19-15.49.10 mnop
2021-12-19-15.54.10 qrst

I want to be able to count the number of rows in the table for every 10 minutes e.g.

Time count
2021-12-19-15.40 2
2021-12-19-15.50 2
2021-12-19-16.00 1

completed_timestamp = Timestamp, details = varchar

I’ve seen this done in other SQL languages but so far have not figured out how to do it with DB2. How would I do that?