As suggested by @CrossRoads, you can use standard TWI Bus Commands (Wire.beginTransmission(deviceAddress);, Wire.write(arg);, Wire.endTransmission();, Wire.requestFrom(deviceAddress, n);, Wire.read();) of the Wire.h Library to read/write the BCD formatted content of the SEC Register of DS1307 RTC. Add 01 (another BCD number) with the content of SEC Register and write it back into SEC Register. (Be sure you are writing BCD formatted value of 20 into SEC Register.) Few hints for you:
1. Check that DS1307 is present and set the address of SEC Register.
2. Read the content of SEC Register. (You have said that it is 19=00011001.)
3. Add 01 with 19; the MCU will produce 1A (0001 1010) and not 0010 0000 (20 BCD).
4. Adjust the incorrect BCD of Step-3 into correct BCD and write it into SEC Register. (If you don't know how to do it, you may take the following alternative steps.)
3. Convert the BCD content of SEC Register into natural binary (normal decimal) just by adding the positional weights of the BCD digits.
4. Add 0x01 with the value of Step-3.
5. Convert the value of Step-4 into BCD value using % (modulus) and / (division) operators.
6. Write the value of Step-5 into SEC Register.
Good luck! We wish to see your full codes in your next post of this thread.