Time Zone Changes

With two recent work trips and a five-city European vacation, combined with the changes to summer time in the US and Europe, I’ve gone through quite a few time zone changes in the past six weeks.

I wanted to find a good way to visualize this, and I ultimately decided on the following format:

Chart showing UTC date (from 6 Mar to 10 Apr 2016) on the X-axis and UTC offset in hours on the Y-axis. A line shows Paul’s UTC offset at every point over the duration of this offset. It starts at UTC−5 in Dayton. On 7 Mar shifts to UTC−6 in San Antonio. On 10 Mar shifts to UTC−5 in Dayton. On 13 Mar, US Daylight Saving Time begins and Dayton shifts to UTC−4. From 19 to 20 Mar, the time zone shifts to UTC−5 in Chicago, briefly back to UTC−4 on an unlabeled layover, then up to UTC+1 in Munich and Nuremberg. On 26 Mar shifts to UTC in London, and then on 27 Mar Europe Summer Time begins and London shifts to UTC+1. On 29 Mar shifts to UTC+2 in Paris. On 2 Apr shifts to UTC in Reykjavík. On 3 Apr departs UTC, and on 4 Apr arrives in UTC−5 in Chicago, and stays in UTC−5 for Tulsa. On 8 Apr, shifts to UTC−4 in Dayton.

Time zones can be challenging to represent, so let’s start with talking about UTC.

UTC (which superseded Greenwich Mean Time, although for our purposes they’re interchangeable) is the time zone of the prime meridian, and it does not change for summer time. UTC is used in situations where an unambiguous, continuous reference time is necessary. For example, flights are scheduled in UTC to avoid confusion about time zones and summer time—effectively, UTC is aviation’s “official” time.

The time zones we’re familiar with (for example, Eastern Standard Time or Eastern Daylight Time) are then represented as differences from UTC. Eastern Standard Time is five hours behind UTC—that is, when it’s 11:00 a.m. in UTC, it’s 6:00 a.m. in Eastern Standard Time. Thus, EST is UTC minus 5 hours, written as UTC−5. In the summer, Eastern time shifts an hour forward to Eastern Daylight Time, so EDT is only four hours behind UTC, or UTC−4.

When I’m flying around between time zones, or switching between standard time and Daylight Saving Time (DST), I’m really just changing the number of hours I am ahead of or behind UTC. So for any given moment in time (as represented by UTC), I can plot my difference in time from UTC.

Thus, in the above chart, the horizontal axis represents UTC, and the vertical axis represents the number of hours I’m offset from UTC. Every horizontal gridline represents a one hour shift. The colored bars represent time I spend in a time zone, and the gray lines between them represent travel.

So, for example, when I flew from Reykjavík to Chicago, I had to set my watch back by five hours (as I traveled from UTC+0 to UTC−5).

You can also see from the chart that I went through two shifts to summer time: I was in the US when they changed to DST on 13 March, and I was in Europe when they changed to summer time on 27 March. In each case, my watch had to move an hour forward, even though I didn’t travel anywhere.

Incidentally, with all this talk about UTC and Greenwich Mean Time, I’m happy to report that my Europe trip included a visit to the Royal Greenwich Observatory in London, which is the point that defined both the prime meridian and Greenwich Mean Time!

Paul straddling a yellow line labeled Prime Meridian at the Royak Greenwich Observatory.