Formula to subtract 1 hour from military time

Hello. I have what seems like a simple request yet I can't seem to figure out the formula. I have a column entitled START TIME formatted as hh:mm AM/PM. In a separate cell on the same sheet I want to take that START TIME, convert it to military time, and subtract 1 hour. From a previous thread I found the formula to convert to military time:

=(VALUE(LEFT([START TIME]@row, FIND(":", [START TIME]@row) - 1)) + IF(RIGHT([START TIME]@row, 2) = "pm", 12, 0)) + ":" + MID([START TIME]@row, FIND(":", [START TIME]@row) + 1, 2)

I've found numerous threads on how to find a duration by subtracting a start time and end time but I can't seem to find a formula to subtract a fixed amount from just the start time. Any suggestions?

Thanks in advance


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