There was a typo. Try

=A1 + TIME(0, 15, 0)

which is equivalent to

=A1 + 1/(24*4)

or

=A1 + 1/96

with the cell(s) formatted as times.

In article ,
Ntisch wrote:

I am getting an error when I enter your formula as;

= A1+Time(),15,0)

Is there something I have left out?