As you can see, I added some test code to the existing example TimeTeensy3. With test set to HIGH you can see that the clock doesn't advance beyond 5 minutes or so.

With test set to LOW the clock advances OKE.

Seems that there is a problem with adjustTime?

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>>> Added code <<<<
boolean test=HIGH;
if (test) {
adjustTime(120); // Does not work beyond 5 min. when test=HIGH
} else {
setTime(now()+120); // Works perfect when test=LOW