Using Millis for keeping time

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hello, everyone, new arduino , first project! in project, using millis() keep track of time isn't accurate. timer 14 seconds slow every hour. know internal clock of arduino isn't exact think 14 seconds big of error. wondering why case. because code long , that's why doesn't keep track of time properly? don't have or while loops block code. appreciated. thank you!

i attached code @ bottom, feedback on helpful well.

long unsigned int time;
long unsigned int btntimer = 0;

long unsigned int ???
use
unsigned long


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