It seems you are not yet able, or didn't know you are able, to read code with an eye to modifying it.
These two lines turn on and off relay1:
Until it is obvious how those lines might be changed to do something for 15 seconds at a certain time, I recommend you do whatever it takes to come to a complete understanding of how those two lines of code do that.
Having said that, it is very not the best way to exploit an RTC and a relay to do something for a small period.
Slog your way through this thread:
tl;dr: when working with variables and time, comparisons for exact matches are fraught. Use earlier and later rather than exactly.
tl;dr: working with hh, mm, and ss is harder than using a single number like seconds since midnight or, oddly, seconds since 1 January 1970.
HTH
a7