sonnyyu:
if you need 2ppm RTC, You could try Temperature-Compensated one.
sample;-
DS32kHz 32.768kHz Temperature-Compensated Crystal Oscillator 0°C to +40°C -/+2.0 ppm
I like the DS3231 and DS3232 I2C RTCs, they include a TCXO with the same spec for almost the same price. DS3234 is an SPI version.