You can do it this simple way :
A1 = Mahi
A2 = NULL(blank)
Select A2 Right click on cell --> Format cells --> change to TEXT
Then put the date in A2 (A2 =31/07/1990)
Then concatenate it will work. No need of any formulae.
=CONCATENATE(A1,A2)
mahi31/07/1990
(This works on the empty cells ie.,Before entering the DATE value to cell you need to make it as TEXT).