The DS1307 and matching crystals are supposed to operate best within a rather narrow range near room temperature. Even the best matched, calibrated DS1307 and crystal will drift when subjected to lower or higher temperatures. That's why the DS3231 is so accurate...not only does it have a built-in, pre-trimmed and calibrated crystal, but it modifies the load capacitance in response to temperature measurements. It also keeps track of how old it is, and applies an aging factor to the load capacitance as well.