spotpear Raspberry Pi RTC module DS1307
https://www.spotpear.com/index.php/in...
Features
Use maxim DS1307 chip
Extends CR1220 button cell backup
Can be operated with shell command
Include a serial sport connector
Programmable square-wave output signal
Consumes less than 500nA in battery-backup mode
Automatic power-fail detect and swit circuitry
Specifications
Accuracy ±20ppm form 0°C to +40°C
Work voltage 5V
battery backup input for continuous time keeping
Real-time clock counts seconds, minutes, hours, day, date, month, and year with leap year compensation valid up to 2100
work temperature 0°C to +70°C
Ports
I2C
2*5 pin 2.54mm connector