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I'd use a helper column: enter the formula =A2+B2 in K2 and fill down.
To determine if a row is flagged, enter this formula in L2 and fill down:
=IF(($K2-MAXIFS($K$1:$K1,$K$1:$K1,"<"&$K2,$D$1:$D1,$D2))<TIME(8,0,0),"Flagged","")
Alternatively, use the formula
=($K2-MAXIFS($K$1:$K1,$K$1:$K1,"<"&$K2,$D$1:$D1,$D2))<TIME(8,0,0)
in a conditional formatting rule of type 'Use a formula to determine which cells to format'.