App note from Holtek on their Real time clock IC HT1382. Link here (PDF)
The HT1382 is a low power real time clock device which includes two serial interfaces: I2C or 3-wire. The interface mode is selected by the selected device type. The device provides both clock and calendar information in BCD format and also includes alarm functions that can be read or setup by an external device, which is usually an MCU and which will be the master device. The calendar is accurate until the year 2099 and includes automatic leap year correction.