@paddykelly Hi, I don't think I've had any issues, certainly not how you've described. I've not done a lot with Macros in excel either, this was my first proper project. I did sometimes find it got a bit sluggish when working with a lot of values. The show we built this for had references for every edit in Night of The Living Dead, about 1100 shots. If we made changes I'd sometimes need to re-load a sheet or quite restart to get it to populate properly.
I have had problems with Sumif before though, you need to be careful when you drag/copy/paste that you're still referencing the right areas. So for example, if in cell C12 you have the formula =SUMIF(A1:A10,"video1",B1:B10) and you copy/drag this to cell C13 it will copy as =SUMIF(A2:A11,"video1",B2:B11). If you copy it to D12 it will give you =SUMIF(B1:B10,"video1",C1:C10)
This is because by default excel treats cell references as 'relative', so when you copy/paste/drag a function it will re-refrence the ranges in the formulas based on their relative position to the new cell. This is really handy when copying a 'total' for example all down one side of a table. It's more of an issue when your references are two dimensional.
Luckily there is an easy way around it. You can make references 'absolute' rather than 'relative' by adding the $ sign to your formulas. Placing a '$' before a column or row reference makes it absolute, so when you copy/paste/drag that value wont change. So, using the same example as before, if in cell C12 you had =SUMIF($A$1:$A$10,"video1",$B$1:$B$10) and copied it to C13 you would still have =SUMIF($A$1:$A$10,"video1",$B$1:$B$10).
You can use '$' selectively to only make a column absolute but keep the rows relative for example by using $A1:$A10.
Not sure if that will fix your issue but might be worth checking. I think Time2Num and Num2Time will return a #VALUE error if they reference any cells that aren't formatted how they expect them to be.
I'm going to be away for a few weeks now so won't be responding very quickly to things. Good luck.