schtasks - Why doesn't this work?

I type this into cmd or in a batch script:

schtasks /change /RP user_password /TN "existing_task" /ST H:M

If /ST H:M has two digits in the hour it says ERROR: The parameter is incorrect.

If /ST H:M has on digit in the hour it says ERROR: Invalid starttime value.

This should work but for some reason doesn't.

How would I go about making this work so it sets the start time of an existing task?