Timekeeping accuracy is the spec that matters. A DS1307 drifts minutes per month; a temperature-compensated DS3231 holds about 2ppm. Battery backup and supply voltage settle the rest.
DS3231/DS3232 temperature-compensated RTCs, the budget PCF8523 and DS1307, the Si5351A clock generator (8kHz–160MHz), and bare 16MHz crystals — matched to drift tolerance and supply voltage.