This is a digital RTC - Real Time Clock made specifically to be used with the Arduino, Raspberry Pi and all other general-purpose microcontrollers which support the I2C interface. The RTC can be used to store and retrieve the current time. Once saved the RTC will use its inbuilt battery to keep time even when the Arduino or the external microcontroller is powered off. The module can keep time for long periods upto 2 years(depending on the battery you use).

The PCF8563 is a CMOS real-time clock/calendar optimized for low power consumption.
A programmable clock output, interrupt output and voltage-low detector are also provided.
All addresses and data are transferred serially via a two-line bidirectional I2C-bus. The maximum bus speed is 400 kbit/s. The built-in word address register is incremented
automatically after each written or read data byte.

Specifications:

Provides year, month, day, weekday, hours, minutes, and seconds based on
32.768 kHz quartz crystal
Clock operating voltage: 1.8 V to 5.5 V
Low backup current; typical 0.25 µA at VDD = 3.0 V and Tamb = 25 °C
400 kHz two-wire I2C-bus interface (at VDD = 1.8 V to 5.5 V)
Programmable clock output for peripheral devices (32.768 kHz, 1024 Hz, 32 Hz and
1 Hz)
Alarm and timer functions
Integrated oscillator capacitor
Internal power-on reset
I2C-bus slave address: read A3h and write A2h
Open-drain interrupt pin
ElectroStatic Discharge (ESD) protection exceeds 2000 V Human Body Model (HBM)
per JESD22-A114, 200 V Machine Model (MM) per JESD22-A115 and 2000 V
Charged-Device Model (CDM) per JESD22-C101
Latch-up testing is done to JEDEC Standard JESD78 which exceeds 100 mA