Sum in dd:hh:mm:ss

If you change shg solution a bit you can also show duration over 31 days

Formula:

=TEXT(DATEDIF(0,SUMPRODUCT(--LEFT(A1:A2, FIND(":", A1:A2) - 1)),"d"),"0:")&TEXT(SUMPRODUCT(--MID(A1:A2, FIND(":", A1:A2) + 1, 8)), "hh:mm:ss")

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little side step
edit: assuming above text formula is put in A3 then follwing formula will allow you to convert this result into a value you can calculate with the outcome,
(this formula van also be used on single dates in your area)

Formula:

=SUMPRODUCT(--LEFT(B6, FIND(":", B6) - 1)) + SUMPRODUCT(--MID(B6, FIND(":", B6) + 1, 8))