Hi,
I am kind of new here, so I do not know whether what I am going to say is of much use to you.
Now the Crystal frequency is also dependent on temperature. ( in fact a crystal can be used as an accurate temperature sensor by using proper circuits).Inside the room where temperature is maintained at 25C it is okay. But for uses which involves long term remote data logging Time accuracy becomes very important.
In such cases TCXO s have to be used.
The cheapest I know is Dallas Semiconductor make 32.768 KHz TCXO.
Costs about $5. This can give 7ppm accuracy/C. There are others of higher accuracy (2ppm) of higher frequencies.
10, 20, 40 Mhz.Most of these take currents of the order of 20 Ma. The Dallas one takes only micro amps.
I hope this note is of some use.
Venugopal