Excel 2013 :: If Statement To Generate Timestamp?

I am using Excel 2013 on a windows 8 machine. It may be easier for me to explain the problem, before posting about the formulas. I maintain a retirement residence with alot of phone numbers. I have a list of all the numbers we own in one column and the numbers assigned to the PBX systems in a second column. By using a If formula I can tell which numbers have not been assigned (The true statement). My problem is when a resident passes away, I need to reuse that "phone number", but I dont want to reuse the number too soon. I am hoping to use the "true" from the if statement to generate a time stamp. I plan on using that time stamp against the "today()" in a days formula, I believe this will spit out the number of days a number has been idle. The time stamp would be constantly changing or removed as the if statement would always be changing. So column C gives the True statement and says "Number not Assigned" - this should create a time stamp in Column W

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