The most popular RTC for the Arduino is the DS1307. However, it does have some drawbacks, the most notable of which is that its operating voltage is 5v, which means it cannot be used with 3.3v projects.
The Maxim DS1339 however, features a wide tolerance of voltages from 2.97V-5.5V with the typical voltage as 3.3v, a battery backup, two alarms, and a trickle charger. The breakout board here packages the DS1339 with the components and connections necessary to use with your Arduino projects easily.
ٖFor more detail: MAX DS1339 RTC REAL TIME CLOCK FOR ARDUINO
