Else IF problems

Hi'

Here's the formula:

If DateDiffInDays({DATE_BT}, CurrentDate()) <= 30 then "0-30" Else

If DateDiffInDays({DATE_BT}, CurrentDate()) > 30 and DateDiffInDays({DATE_BT}, CurrentDate()) <= 90 then "30-90" Else

if DateDiffInDays({DATE_BT}, CurrentDate()) > 90 and DateDiffInDays({DATE_BT}, CurrentDate()) <= 120 then "90-120" Else

if DateDiffInDays({DATE_BT}, CurrentDate()) > 120 then "+120"

It keeps telling me "No viable alternative at "If, try rewriting the formula".

Should I be moving the DateDiffInDays formula to another calculated field and use that filed instead in this calculation?

Thanks

Using Lumira Desktop Enterprise 1.29.3