Hello Davo

Your column B isn't the mm:ss that you think it is. It is actual time based on the 24 hour clock, ie: B2 is 9:05 PM.

In C2 put =TIME(0,LEFT(B2,LEN(B2)-3),RIGHT(B2,2)) *24*60 to get minutes with a decimal, drag down, then sum the column as normal.

Good luck
NoSparks