DS1307 Real-Time Clock (RTC) Module

Keeping accurate time across a power cycle is a problem most microcontrollers can't solve on their own, since their internal clocks reset the moment power drops. The DS1307 solves this with a dedicated timekeeping chip backed by its own small coin-cell battery, so the date and time keep ticking along even while the rest of the project is completely powered off, and firmware simply reads the current time back over I2C on the next boot. Compared to the newer DS3231, it skips the temperature-compensated crystal that keeps that chip accurate to a few seconds a month, trading a little long-term drift for a lower price, which makes it a sensible choice for clocks, data loggers, and scheduled-event projects where being off by a minute or two over several months isn't a problem.