In the schematic there is a button labelled S4 that is connected to PD3 (D3 on Arduino) but it seems that in the sketch there is not any code about it.

It seems that in the sketch there is no code to enable the square wave output from DS1307.

In initclock() I added these

  //Enable Square Wave Output
  Wire.beginTransmission(DS1307_I2C_ADDRESS);
  Wire.write(0x07); // move pointer to SQW address
  Wire.write(0x10); // sends 0x10 (hex) 00010000 (binary)
  Wire.endTransmission();

and a

define DS1307_I2C_ADDRESS 0x68

at the beginning

I made a porting for cheap OLED display here
https://github.com/zoomx/OLED-Pong-Clock