Hi Karthick,
Use the below DAX code -
datediff= IF(ISBLANK(table[Completed date].[Date]) = "TRUE", DATEDIFF(table[estimated date],TODAY(), DAY), DATEDIFF(table[completed date], table[estimated date], DAY))
The above code calculate the days difference i.e, estimated date - completed or todays date.
Hope it helps you.