The system time can be updated, where applicable, by means:
I2C RTC
MCP794x (for example: http://bitwizard.nl/shop/I2C-RTC)
DS3231 (for example https://www.adafruit.com/product/3013)
NTP Client (see Network configuration)
Art-Net ArtTimeSync
UART GPS time
ATGM336H (or compatible)
ublox-NEO7
MTK3339 {for example: https://www.adafruit.com/product/746}
When there is none of the above, the system time is the uptime.