You haven't called the method, you've compared it!
To call the method, which is what you want, do this: rtc.getTimeStr()
string constants are surrounded by double quotes: "20:00:00"
If it returns a string, use strcmp. http://www.cplusplus.com/reference/cstring/strcmp/
if (strcmp (rtc.getTimeStr(), "20:00:00") == 0)
{
..
}
This still won't do what you want, of course, as the test will fire every time round loop for a whole second
(which could be thousands of times). Study the StateChangeDetection example in the IDE.