I have table with datetime column in datasource. I want to calculate difference between first row datetime with second row of datetime.

[ankit_vaghela]

NULL

[swapnil_aswale]

As far as I know unlike excel power BI works only with column reference in table format and not row reference, hence it will not allow calculations in horizontal mode. You can transpose the data and then calculate the difference and re-transpose in Query Editor. Hope this will help you.