How to display a default value for minute field of Time fieldtype
As the time component says in its description field, it "accepts a time in any GNU Date Input Format. Strings such as now, +2 hours, and 10:30pm are all valid."

If you want to set the default minute, you can used a formatted string like "1972-09-24T20:02:00,000000-0500" or "10:30pm" in the Default value field.