IF Function when Blank

[datastudent]

Hi,

I want to remove this "more than 90 days" in my Tenurity column when the Promotion is blank or 0.
The result I want to show:
less than 90 days
90 days
more than 90 days
blank(if no promotion #)

Here's the formula I tried so far.
=IF(@Q:Q<=89,"less than 90 days",IF(@Q:Q=90,"90 days",IF(@Q:Q>=91,"more than 90 days",IF(ISBLANK(@Q:Q),""))))
=IF(@Q:Q<=89,"less than 90 days",IF(@Q:Q=90,"90 days",IF(@Q:Q>=91,"more than 90 days",IF(@Q:Q=0,"external"))))

 

[Fluff]

How about

=IF(Q2="","",IF(Q2<=89,"less than 90 days",IF(Q2=90,"90 days","more than 90 days")))