Calculated Column - complex formula

Question

One of our previous SharePoint Developer developed this formula for a calculated column named as "pGantt" in a SP2010 document library. I am not able to understand its syntax. The value produced by the formula is in always percentage eg. 130%, 110%, 90%, etc. Kindly explain what exactly the formula is meant for.

Many thanks in advance.

Here is the code: =IF(ISERROR(IF(ISERROR(DATEDIF(CommencementDate,Today,"m")),0,DATEDIF(CommencementDate,Today,"m"))/IF(ISERROR(DATEDIF(CommencementDate,ExpiryDate,"m")),0,DATEDIF(CommencementDate,ExpiryDate,"m"))),1,IF(ISERROR(DATEDIF(CommencementDate,Today,"m")),0,DATEDIF(CommencementDate,Today,"m"))/IF(ISERROR(DATEDIF(CommencementDate,ExpiryDate,"m")),0,DATEDIF(CommencementDate,ExpiryDate,"m")))

Answer 1

It calculates

(Months from CommencementDate to today)/(Months from CommencementDate to ExpiryDate)

Ie how expired the document is.

Most of it is for handling problems with data.