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The schedule "start-time" argument is optionally passed when adding or removing an image snapshot schedule or a trash purge schedule. It is supposed to schedule an initial operation at a specific time of day for a particular time zone. Instead, it offsets the schedule’s anchor time by the specified "start-time". By default, the scheduler uses the UNIX epoch as the anchor to calculate recurring schedule times. The "start-time" argument simply shifts this anchor away from the UTC epoch, which is confusing. For example:

$ # current time
$ date --universal
Wed Dec 10 05:55:21 PM UTC 2025
$ rbd help mirror snapshot schedule add
usage: rbd mirror snapshot schedule add
                                      [--pool <pool>] 
                                      [--namespace <namespace>] 
                                      [--image <image>] 
                                      <interval> [<start-time>] 

Add mirror snapshot schedule.

Positional arguments
  <interval>           schedule interval
  <start-time>         schedule start time

Optional arguments
  -p [ --pool ] arg    pool name
  --namespace arg      namespace name
  --image arg          image name
$ # schedule interval of 1h and start-time at 19:00UTC
$ rbd mirror snapshot schedule add -p data --image img1 1h 19:00Z
$ rbd mirror snapshot schedule ls -p data --image img1
every 15m starting at 19:00:00+00:00

You would expect that the scheduler will run the first snapshot each day at 19:00 UTC and then run snapshots every 15 minutes. Instead, the scheduler runs the first snapshot at 18:00 UTC and then runs snapshots every 15 minutes:

$ rbd mirror snapshot schedule status -p data --image img1
SCHEDULE TIME        IMAGE
2025-12-10 18:00:00  data/img1