That approach was for use in conjunction with Custom Format approach, if using formula I would opt for:
=IF(MOD(G91-F91,1)<=0+"00:30","ONTIME",MOD(G91-F91,1)-"00:30")
change the "00:30" to be whatever time span you want (but yes in essence the value provided was a rough representation of 30 mins in numerical terms, 24 hours = 1)
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