Here's one of the complicated solutions. It's probably best just to paste this in and hope for the best rather than trying to sit down and parse this.

```
=INT(MOD(MID(B2,FIND(":",B2,FIND("-",B2))-2,2) - LEFT(B2,FIND(":",B2)-1)+(MID(B2,FIND(":",B2,FIND("-",B2))+1,2) - MID(B2,FIND(":",B2)+1,2))/60+IF(MID(B2,FIND("m",B2)-1,1)=MID(B2,FIND("m",B2,FIND("-",B2))-1,1),IF(MID(B2,FIND(":",B2,FIND("-",B2))-2,2) - LEFT(B2,FIND(":",B2)-1)+(MID(B2,FIND(":",B2,FIND("-",B2))+1,2) - MID(B2,FIND(":",B2)+1,2))/60<0,24-IF(LEFT(B2,2)="12",12,0),0),12-IF(AND(MID(B2,FIND(":",B2,FIND("-",B2))-2,2) - LEFT(B2,FIND(":",B2)-1)<>0,MID(B2,FIND(":",B2,FIND("-",B2))-2,2)="12"),12,0)),24)) & ":" & TEXT((MOD(MID(B2,FIND(":",B2,FIND("-",B2))-2,2) - LEFT(B2,FIND(":",B2)-1)+(MID(B2,FIND(":",B2,FIND("-",B2))+1,2) - MID(B2,FIND(":",B2)+1,2))/60+IF(MID(B2,FIND("m",B2)-1,1)=MID(B2,FIND("m",B2,FIND("-",B2))-1,1),IF(MID(B2,FIND(":",B2,FIND("-",B2))-2,2) - LEFT(B2,FIND(":",B2)-1)+(MID(B2,FIND(":",B2,FIND("-",B2))+1,2) - MID(B2,FIND(":",B2)+1,2))/60<0,24-IF(LEFT(B2,2)="12",12,0),0),12-IF(AND(MID(B2,FIND(":",B2,FIND("-",B2))-2,2) - LEFT(B2,FIND(":",B2)-1)<>0,MID(B2,FIND(":",B2,FIND("-",B2))-2,2)="12"),12,0)),24)-INT(MOD(MID(B2,FIND(":",B2,FIND("-",B2))-2,2) - LEFT(B2,FIND(":",B2)-1)+(MID(B2,FIND(":",B2,FIND("-",B2))+1,2) - MID(B2,FIND(":",B2)+1,2))/60+IF(MID(B2,FIND("m",B2)-1,1)=MID(B2,FIND("m",B2,FIND("-",B2))-1,1),IF(MID(B2,FIND(":",B2,FIND("-",B2))-2,2) - LEFT(B2,FIND(":",B2)-1)+(MID(B2,FIND(":",B2,FIND("-",B2))+1,2) - MID(B2,FIND(":",B2)+1,2))/60<0,24-IF(LEFT(B2,2)="12",12,0),0),12-IF(AND(MID(B2,FIND(":",B2,FIND("-",B2))-2,2) - LEFT(B2,FIND(":",B2)-1)<>0,MID(B2,FIND(":",B2,FIND("-",B2))-2,2)="12"),12,0)),24)))*60,"00")
```

Edit:
Fixed the formula for the last time.

Addressing comment:

To add these output, to add a range of these output you can use the following formula:

```
=TEXT(SUM(VALUE(B2:B3)),"h:mm:ss")
```

You must enter this as an array formula. Do so by pressing Ctrl+Shift+Enter when entering the formula.