Help! Date Difference Mulitple entries in rows

Hi

First time so please go gentle....

What i need to accomplish is work out the differnce between two date in a table, which keeps getting updated with new data against the same Asset Number.

As you can see we have multiple rows with the same Asset SN as we are taking measurements everymonth... some are misssing so I need to calculate the difference between the two dates to work how long the dates havent been updated

Column E =IF(C2=MAX(IF(A2=$A$2:$A$4859,$C$2:$C$4859,"")),"Latest","old data")
Column F =IF(MAX(INDEX((A2=$A$2:$A$10000)*$C$2:$C$10000,))=C2,"Newest Data","Old Data")
Column H =IF(F15="Newest Data",90-(TODAY()-C15),"")

I hope you can help!!

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