12:00:00 x
10:00:00
13:00:00 x

=SUMIF(B:B, "=x", A:A)

=SUMPRODUCT((A1:A100)*(B1:B100="x"))

=TEXT(SUMIF(B:B, "=x", A:A), "[h]:mm:ss")

=TEXT(SUM(FILTER(A:A, B:B="x")), "[h]:mm:ss")

=ARRAYFORMULA(TEXT(SUM(IF(B:B="x", A:A, )), "[h]:mm:ss"))

=TEXT(SUM(QUERY(A:B, "select A where B ='x'")), "[h]:mm:ss")

=TEXT(SUMPRODUCT((B:B="x")*(A:A)), "[h]:mm:ss")