Finding average time between list of timestamps

If your timestamps are in a valid time format then you should be able to just subtract one from the other, so if they start in A2 try this formula in B3 copied down

=(A3-A2)*86400*1000

that will give you the difference in milliseconds (format B3 as general).

Copy that formula down the column and then you can just use a regular AVERAGE formula to average the differences.

If you want you can get the average without even computing the individual differences because that will just be the difference between then first and last values divided by the total number of values -1, i.e.

=(MAX(A:A)-MIN(A:A))*86400*1000/(COUNT(A:A)-1)

It's actually two columns, Date > Time and around 200k rows. I'm trying to get it to time format it will recognize.

OK, you can still that with a small adjustment, i.e. assuming dates in A2 down and times in B2 down use this formula in C2

=(A3+B3-A2-B2)*86400*1000

If you select C2 and put the cursor on the bottom right of the cell until you see a black "+" (that's the "fill-handle") you can double-click and the formula will populate as far down as you have data in the adjacent column

Now just average column C

...or once again you can get the average difference with a single formula

=(MAX(A:A+B:B)-MIN(A:A+B:B))*86400*1000/(COUNT(A:A)-1)

confirmed with CTRL+SHIFT+ENTER

The above works with valid dates and times no matter what format. You can check if the data is valid by using

=ISNUMBER(A2+B2)

If that gives TRUE then the formulas above should work

If row 1 contains a text header then that second formula might give you problems. You can try specifying the exact ranges like

=(MAX(A2:A200000+B2:B200000)-MIN(A2:A200000+B2:B200000))*86400*1000/(COUNT(A:A)-1)

Needs to be entered with CTRL+SHIFT+ENTER so that curly braces like { and } appear around the formula.

....but of course you don't need that one if the first formula worked.......