Excel Timestamp Formula

I'm trying to subtract timestamps in Excel that are formatted as "day hour/minute/time."  The problem is that Excel doesn't recognize it as a date or time, so I can't extract out the time calculations. I tried using the "Custom" format to help Excel recognize it as a day/time format, but it's not working.

Example:

12 03:45:12 (day hour:min:sec)

12 03:55:13 

If I want to subtract the first timestamp from the second, how do I do that to arrive at the answer (0 days, 0 hours, 10 min, 1 sec)? I have a feeling the "12" is confusing the formula but I don't know how to subtract it out, as Excel doesn't recognize it as a date without the month and year.