User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:51.0) Gecko/20100101 Firefox/51.0 Build ID: 20170125094131 Actual results: I am using Thunderbird/Lightning extension and I am always correcting the file "zones.json" manually after each update of Lightning extension. The corrected records are as follows; Old and wrong lines: "Asia/Istanbul": { "ics": "BEGIN:VTIMEZONE\r\nTZID:Asia/Istanbul\r\nBEGIN:DAYLIGHT\r\nTZOFFSETFROM:+0200\r\nTZOFFSETTO:+0300\r\nTZNAME:EEST\r\nDTSTART:19700329T030000\r\nRRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU\r\nEND:DAYLIGHT\r\nBEGIN:STANDARD\r\nTZOFFSETFROM:+0300\r\nTZOFFSETTO:+0200\r\nTZNAME:EET\r\nDTSTART:19701025T040000\r\nRRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU\r\nEND:STANDARD\r\nEND:VTIMEZONE", "latitude": "+0410100", "longitude": "+0285800" "Europe/Istanbul": { "ics": "BEGIN:VTIMEZONE\r\nTZID:Europe/Istanbul\r\nBEGIN:DAYLIGHT\r\nTZOFFSETFROM:+0200\r\nTZOFFSETTO:+0300\r\nTZNAME:EEST\r\nDTSTART:19700329T030000\r\nRRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU\r\nEND:DAYLIGHT\r\nBEGIN:STANDARD\r\nTZOFFSETFROM:+0300\r\nTZOFFSETTO:+0200\r\nTZNAME:EET\r\nDTSTART:19701025T040000\r\nRRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU\r\nEND:STANDARD\r\nEND:VTIMEZONE", "latitude": "+0410100", "longitude": "+0285800" Corrected lines: "Asia/Istanbul": { "ics": "BEGIN:VTIMEZONE\r\nTZID:Europe/Istanbul\r\nBEGIN:STANDARD\r\nTZOFFSETFROM:+0300\r\nTZOFFSETTO:+0300\r\nTZNAME:TRT\r\nDTSTART:19700101T000000\r\nEND:STANDARD\r\nEND:VTIMEZONE", "latitude": "+0410100", "longitude": "+0285800" "Europe/Istanbul": { "ics": "BEGIN:VTIMEZONE\r\nTZID:Europe/Istanbul\r\nBEGIN:STANDARD\r\nTZOFFSETFROM:+0300\r\nTZOFFSETTO:+0300\r\nTZNAME:TRT\r\nDTSTART:19700101T000000\r\nEND:STANDARD\r\nEND:VTIMEZONE", "latitude": "+0410100", "longitude": "+0285800" I am not sure whether this correction is fully O.K. or not but it works for me. Please note that, last year (2016) there is a new legislation in Turkey, the time zone is fixed as UTC+3 and there is no Daylight Saving Time anymore. Best regards. Bora

It seems this was already fixed with https://hg.mozilla.org/comm-central/rev/cd64861e22c692f2237e60b141b113ec0a8d0665 in Bug 1266424 for Lightning 5.4 and newer.

Turkish time problem exist current community version TB 45.7.1, Lightning 4.7.7.1