Rob
Timers fire in the main event loop when your all of your code has returned. Long lived code will certainly block the execution of timers (and other mainloop events) The thing that’s surprising is that in your case only one timer was blocked. I wonder if there was actually some other interaction occurring between your logging code and the timer code???
I hope you will use System.Debuglog in the future. Contrary to a TextArea it does not drag on the apps execution, just the IDE.
@Joe Huber Yes, it displayed the time in 4 different offsets in labels and another timer which stopped working after 4-5 hours did execute several methods to gather info from the internet, process the info and store it in a database.
I did fall back to Xojo 2015 r.4 and now the timers do stop working after 14-15 hours executing, which could be related to the issue found, because there are no counters used in these methods which goes out of bounderies or cause other faults. The application showed like it is still working, but info is not dynamic anymore…I mean, does not change anymore, till I stop/start the tool again.
@Michel Bujardet , I do store the system events in a syslog now. This info contains different info like loss of connection, invalid logins, log rotation, file download failures, etc.
At this moment everything looks good as long as the timers keep on running 
I think Xojo found the issue with the timers in the dumps I send them, because they closed my case and related my case to an earlier case in which this issue was investigated and a fix was created which will be tested soon. (maybe will be tested already)