DS1307 + Coin-Cell RTC

Add a battery-backed real-time clock to any Arduino with two wires and a coin cell, then read date and time over I²C.

Difficulty: Breadboard. Estimated build time: about 30 minutes. Estimated parts cost: about US$5.00. 6-part bill of materials with live supplier pricing. A real-time clock you build by hand on a breadboard, not a pre-soldered module.