The PIC doesn't have an "internal resonator", it has an internal R-C oscilator (some require extrnal parts) an RC oscillator is much less acurate than a resonator or crystal. It also has a much greater temperature coeficient. Meaning that the speed of the osc will be related to the ambient temperature. And yes, it could run fast and slow depending on the temp.
If the place where you intend to install this has a mostly steady temp, then you could probably get by with a "fudge factor" and get within a couple minutes a day. But remember the error will be cumulative. One minute a day is half an hour a month, six hours a year. To keep it near midnight you'll have to reset it about every 6 months.
-Denny
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