SQL & PL/SQL :: Query To Get Timestamp From Only One Date Column?

I want to write a query to get the time stamp from only one date column,

I tried using a group by clause but getting error "not a group by exp."

Below is the query

SELECT ProdID,ProdRequestID, SUBSTR((max(EVENTTIMESTAMP) - min(EVENTTIMESTAMP)), 18,2)Execution_Time
FROM LOG_TIMESTAMPS where ProdID = 1680988889
group by ProdRequestID
ProdID||ProdRequestID ||EVENTTIMESTAMP

[code]....

In the above i am looking for a diference on ProdRequestId,

Output

ProdIDProdRequestIDEVENTTIMESTAMP
16809888892013-04-09T02-56-34.5025kqcxy03