If long term total accuracy is required, and your device (as Yet unspecified) is line operated I would go with a generator chip, and syncronize it with the power utility the accuracy over one day is less than 20 cycles , but the long term accuracy is far better than any stand alone crystal. Note this only applies to grid linked systems. My field was stand alone or assist generators so I never worked real close with the grid control group, but It is a very inexpensive long term accurate signal if available. Terry