How to derive the “End of Evaluation Date” value in RFx

Hi all,

I would like calculate the number of days spent performing an RFx evaluation, and am wondering if you can provide me a clue on the formula.

          |----------------------------------------------------------------------------------|

(Start of evaluation Date)                                         (End of evaluation Date)      

Hence,
     Days spent evaluating RFx = End of Evaluation Date – Start of Evaluation Date.   

To derive the “Start of Evaluation Date”, I will select the date when the RFx header status = “Ended RFx” (i.e. this is the date when evaluation of RFx starting).

Question:

How to derive the “End of Evaluation Date” value? 

Thanks
Tuff