Time display / non display.

Hi,

Looking for help again..., I have 2 columns E and F that I am using, in Column E, I enter a time say 22:43:00, column F currently ads 15 minutes to what ever is in there with '=E3+TIME(0,15,0)' which is fine except if column E
has no time in it when it automatically puts in 00:15:00, hov can I get the cell not to display anything in there is no value in column E.

As usual, thank you in advance