# Time

The fourth dimension, at least in theory.  We keep track of time in a linear way, each second marking a particular point that will happen only once and never be revisited.  There will never be another 10:42:30.1 on April 17, 2013, for example.

Point in space and time

Of course there are several ways to record the same time:

• Coordinated Universal Time (UTC): Which is the time at the prime meridian, 0° Longitude.  From there, time zones span out to +12 or -12 UTC, meeting again at the International Date line.  In military parlance, UTC is known as Zulu because it is in timezone Z.
• Local Time: In any given location, is determined when the sun is directly overhead (± sidereal correction) at noon.
• Local Timezone:  One of twenty four arbitrary divisions where the sun may be directly overheard (± sidereal correction) somewhere within the division at noon.
• Unix Timestamp:  The number of seconds that has transpired since 0000, January 1, 1970.  Unix time stamp 1366209730 equals 10:42:30, April 17, 2013.  In hex looks like 516F0260.  Used by all Unix/Linux variants.
• GPS Time: UTC – LS (Leap Second) + 19 s.
• ISO 8601 date/time: 2013-02-17T10:42Z
• Julian Date: A continuous count of days and fractions of such since noon Universal Time on January 1, 4713 BCE.  April 17, 2013 10:42:30.1 equals 2456399.946193

One thing to note and mark your calendars: Unix (and variants) may have a problem on January 19, 2038 because of a 32 bit integer issue.  This is known as Y2038, and a smart man would start planning now.

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• Bob M.

I suppose there will be a lot of code fixed in the next 25 years, but how many of us will be around to worry about it. Will anyone still be using *nix or will Microsoft have bought it out and killed it, just to reinvent it and claim it was their idea. I wonder what problems their timestamps have?

Bob M.

• Time keeps on tickin’, tickin’, tickin’…

• Alan Peterson

Very true that there will never be another 10:42:30.1 on April 17, 2013, but if you’re fast, you can watch the sun set twice in the same day.

Years ago when I lived in Oswego NY (first job, WSGO radio), I joined a bunch of college students on the shore of Lake Ontario, waiting for sunset. When the top edge of the sun dipped into the water, we all scrambled up the shallow dropoff/cliff – just quick enough to be ahead of the light’s edge – and watched the last bit of sun drown once again.

Anyone who lives near skyscrapers can enjoy the same phenomenon, if the express elevators to the observation deck are fast enough.

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