Hello Stanley,
Your crystal oscillator is the root of your problem. The best thing you can do is to change the timing constant in your timer software until the error becomes acceptable. This will take a few days of tinkering to accomplish.

Even after you get it the best you can it will not be acceptable because the error will accumulate over time and become unacceptable.

What you need is a timing source which has this error corrected automatically. Fortunately there is such a source. It is utility you get your power from. Use that as your timing source and you will get better accuracy.

Regards,

Charles Bannister