Stored Procedure

I am attempting to create a stored procedure for a database that will take a time value, such as "12:00:00 PM" and subtract another value from it, such as "0:01:23" and then input the answer into a record in the format of the first value, in this case "11:58:37 AM" would be the hopeful outcome. I am new to stored procedures, so I am not exactly sure where to begin. If anyone knows of any links that could help me find a solution to this or if anyone has a solution, I would appreciate the info.

Thanks!