Thanks for the replies. I tried pylon's code and I get the error: no match for 'operator+=' (operand types are 'DateTime' and 'TimeSpan')
Excuse me, I expected the compiler to do a better job :).
Replace the line
now += TimeSpan(3600);
by
now = now + TimeSpan(3600);
which is the same but here the compiler has the exact operator for matching.