A bug has been found in the Asterisk 13 code that prevents Vicidial from detecting actual time synchronization problems. Meaning if two servers in a clustered install of Vicidial have different times, Vicidial will not be able to detect this and will not alert you of the issue. It will only alert you if Asterisk has crashed on the server. If you are using Asterisk 13 (and you should be by this point) it is recommended that you upgrade to at least SVN revision 3330 of the Vicidial code base to get the fix. Without this code functioning properly it can be nearly impossible to realize that there is even an issue with the time on your servers. It however can cause problems where calls fail to route to agents. It can also cause problems with phone calls showing up with negative call lengths.
Please note that this bug will NOT cause time synchronization problems. Those are caused by either hardware issues or operating system configuration problems. It simply fails to detect when this is happening.