I have this spreadsheet in the picture... I want to fill up to 30/ago but the only way I know is to fill by hand...
What would be the formula to drag this horizontally and it increment by 1 per column?

You can use:
=COLUMN() & "/ago"
it returns 1/ago in first column, 2/ago is second and so on.
Also you can adjust formula, say:
=(COLUMN() -3) & "/ago"
returns 1/ago in column 4, 2/ago in column 5 and so on.

Another way: in starting cell, say B4 wirte 1/ago. Then in next cell, C4 in our case, write
=COUNTA($B$4:B$4)+1 & "/ago"
and drag across
