I have recently installed 18:04 and run Thunderbird 60.2.1 with lighting 126.96.36.199.
It looks like the Lightning Calendar is using a different time than my system time. I recently migrated to a new computer and copied my old Thunderbird profile to the new computer. However, the migration to the new computer took place in a different time zone than my old profile was used, and Thunderbird / Lighting does not appear to have been updated to the required time zone.
For example, I am now on the East Coast and behaves through the lightning-time zone as on the west coast. If I add a new event in the future for 2 minutes and tell them there should be a notification 0 minutes in advance, no notification will be displayed if the time zones are correct. However, if I add the event for 3 hours in the past (my old time zone and the old profile I copied), a pop-up notification will be displayed for the event.
Why is Lightning using a different time zone than my system time zone and how do I change it?