Why make thing complicated???
=HOUR(TIMEVALUE(TRIM(MID(SUBSTITUTE(SUBSTITUTE(A1,"AM"," AM"),"PM"," PM"),11,25)))) - MOD(HOUR(TIMEVALUE(TRIM(MID(SUBSTITUTE(SUBSTITUTE(A1,"AM"," AM"),"PM"," PM"),11,25)))),2)
It's very simple formula:
=HOUR(A1)-MOD(HOUR(A1),2)
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