chrony is a versatile implementation of the Network Time Protocol (NTP). It can synchronise the system clock with NTP servers, reference clocks (e.g. GPS receiver), and manual input using wristwatch and keyboard. It can also operate as an NTPv4 (RFC 5905) server and peer to provide a time service to other computers in the network.

Blog mostly about the network time protocol (NTP).

Raspberry Pi add-on board for Smart Power Management and additional GPIO.

Run Raspberry Pi on Battery Power
Extend RPi battery life from hours -> weeks / months
Deep Sleep, with Wake on Event or Time
On-board Arduino for user programs
Adds Arduino Analogue & PWM IO
Battery-backed Real-Time clock / calendar
Avoid SD Card corruption with proper shutdown
Works with all 40-pin Raspberry Pi Models (Zero, A+, B+/B2 and B3)

Module with uBlox NEO-M8T and antenna.

The RADclock project (formerly known under 'TSCclock') aims to provide a new system for network timing within two years. We are developing replacements for NTP clients and servers based on new principles, in particular the need to distinguish between difference clocks and absolute clocks. The term RADclock, 'Robust Absolute and Difference Clock', stems from this. The RADclock difference clock, for example, can measure RTTs to under a microsecond, even if connectively to the time server is lost for over a week!
The RADclock is available and can be used as an alternative to the ntpd daemon to provide convenient system clock synchronisation for systems running FreeBSD and Linux. In terms of local hardware, it exploits the availability of counters such as the HPET counter or the TSC (Time-Stamp-Counter) commonly found in PC architectures.

A battery-powered real-time clock for the Raspberry Pi.

Backup strategy based on rsync and ln.

Statistics of an NTP server.

Poul-Henning Kamp of FreeBSD fame uses an array of GPS receivers for timekeeping.