Time Zone Visualizations
I’ve been doing a lot of work with time zones recently; they’re notoriously difficult to wrap one’s head around, so I created a few visualizations to help me.
UTC Offsets
The first visualization shows the concept of UTC offsets, by showing midnight on New Years Eve 2008 as it occurred across several time zones. The zones are lined up so that any vertical line will intersect simultaneous times in each zone:
Eastern Standard Time has a -5 hour offset, which means that 12AM EST is simultaneous with 5AM UTC (12AM + 5 hours = 5AM). Calculating the other way, 12AM UTC is simultaneous with 7PM EST (12AM – 5 hours = 7PM).
Daylight Savings: Spring Forward
Some time zones observe daylight savings time, and therefore have two UTC offsets: the standard offset, and the DST offset. The US Eastern time zone, for example, has a -5 hours standard offset (Eastern Standard Time), and -4 hours daylight savings offset (Eastern Daylight Time).
The next visualization shows the “spring forward” period, where standard time becomes daylight time:
Because of daylight savings, on Mar 9, 2008 in the US Eastern zone, 2AM never occurred, because the instant the clock hit 2AM, it immediately sprang forward to 3AM. Of course, no actual jump in time occurred — time proceeded linearly; it was only the hh:mm:ss labeling of time that experienced an adjustment.
Given that UTC does not adjust itself for daylight savings, when you move the clock forward an hour for daylight savings, you’re also moving the UTC offset forward one hour. So, US Eastern time has a UTC offset of -4 hours during daylight savings.
Daylight Savings: Fall Back
On Nov 2 2008 in the US Eastern zone, when the clock strikes 2AM, it will roll back to 1AM:
… so there will actually be two 1AMs during that day: 1AM EDT, followed one hour later by 1AM EST. “Nov 2 2008 1AM” is therefore an ambiguous time point identifier; to disambiguate, one would need to specify either daylight savings time or standard time.
Daylight Savings: Full Year View
The last visualization shows an entire year, comparing two DST-observing US zones with UTC:
It’s worth noting that despite the springing forward and falling back, any time point in any time zone has exactly one simultaneous time point in any other time zone.
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