I'm tired and concede this is hideous but I feel sufficiently guilty for posting an untested solution that I hereby provide you with another...
Using my prior upload the following
L13:
=IF($D13<=$E13,MAX(0,(MIN(L$12,$E13)-MAX(L$11,$D13))*MIN($D13<(L$12-"0:15"),$E13>(L$11+"0:15"))),((L$12-MAX(L$11,$D13))*($D13<(L$12-"0:15")))+((MIN(L$12,$E13)-L$11)*($E13>(L$11+"0:15"))))
applied across L13:Mx (x being last row)
should work I think... I will revisit tomorrow.
(change from prior version is highlighted in red)
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