Re: [guide-user] Time zone TDT

Bill J Gray Jan 12, 2012

Hi Luc,

Uh oh... this is going to require more thought than I at first realized.

Here's what I've figured out thus far:

If you've selected Dynamical Time = TDT, or UT (*), or UTC, then it
makes sense that all times should reflect that "time zone", whether shown
in day/month/year/hour/minute/second (DMYHMS) form or as a JD or MJD. This
applies both to times entered into Guide and times displayed by Guide.

If your time zone is, say, Eastern US standard time (EST), which is
five hours behind UTC, things really ought to be different. At 7:00 PM
EST last night, it was midnight UT. So

2010 Jan 10 19:00:00 (EST) = JD 2455937.50000 (UTC) = MJD 55937.00000 (UTC)

I've never seen something like a "JD (EST)". Julian Days (and Modified
JDs) are, at least in my experience, either UT, UTC, or TDT.

My current thought is: if the current "time zone" is Dynamical Time,
Guide should show JDT and assume any Julian or Modified Julian entries
are on Dynamical Time (and Guide should show the abbreviation "JDT" rather
than "JD"). For all other "zones", Guide should show a JD based on
Universal Time. Probably a UTC one.

-- Bill

(*) Pre-emptive note to pedants: by 'UT', I mean 'UT1', the Universal
Time we would have on an earth that didn't have polar motion. Guide
doesn't attempt to compute UT0, the "true" Universal Time that is
location-dependent (despite the word "Universal").