I have an iMac and a MacBook Air, both running OS X Yosemite 10.10. Both set the time zone, date and time automatically. I noticed recently that they display slightly different times. When I checked, the iMac showed 14.43.14 while the MacBook said 14.42.34. www.timanddate.com said it was 14.38.30. I looked at my iPhone 5 (iOS 8.1.2) which told me the time was a full five minutes earlier than the MacBook. All devices are running on the same wifi network. I have tried all of the obvious things like re-starting, checking that location services are on, but no luck. I tried to find the invisible folder etc/localtime, but could not find it, so if that is involved please let me know where it is hidden. Any advice would gratefully received.