It’s March 2nd, 1943. The user asks your software to schedule a meeting with Acmecorp at “9 AM on the first Monday of next month”.
[6:17:45] homura ~ $ cal -3 2 March 1943
February 1943 March 1943 April 1943
Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa
1 2 3 4 5 6 1 2 3 4 5 6 1 2 3
7 8 9 10 11 12 13 7 8 9 10 11 12 13 4 5 6 7 8 9 10
14 15 16 17 18 19 20 14 15 16 17 18 19 20 11 12 13 14 15 16 17
21 22 23 24 25 26 27 21 22 23 24 25 26 27 18 19 20 21 22 23 24
28 28 29 30 31 25 26 27 28 29 30
Right now, California is on Pacific Standard Time (PST) and Arizona is on Mountain Standard Time (MST). On March 8th, California will transition to Pacific Daylight Time (PDT), one hour ahead. Arizona does not observe DST, so they’ll stay behind.
At least until April 1st — when the governor will sign an emergency order moving the state to MDT, effective immediately.
Back on March 2nd, you send an email to each participant telling them about the meeting. One of them has their locale set to en_GB, so some of the participants need to be sent “04/05/43” and some “05/04/43”.
A moment later, the user asks you to tell it the number of hours betweeen now and the meeting they just scheduled. The subject of the meeting is purchasing fuel for a machine that the user is now filling with enough fuel to last until then.
On the day of the meeting, the user drives to the Navajo reservation to conduct some unrelated business, and has to attend the meeting by phone. The reservation has been on daylight savings time since March 8th, by the way, they never stayed behind with the rest of Arizona. The user expects the software to warn them 1 hour prior to the meeting start. The border of the reservation is defined by a river, which is slowly moving East.1
The changelog for the IANA zoneinfo database is great, by the way, you should read it. Or subscribe to get it periodically2 material delivered to your inbox!