Why do DateList, DateString, and AstronomicalData ignore $TimeZone?

Question

My reading of the documentation for DateList and DateString lead me to believe that both functions use $TimeZone to determine the time zone to use in interpreting their inputs. Thus, for example, if the local machine time zone is UTC-5, $TimeZone is -5, and the time at Greenwich, corresponding to a given time in the local time zone can be found by specifying TimeZone -> 0 as an option:

DateList[{2012, 10, 8, 7}, TimeZone -> 0]
{2012, 10, 8, 12, 0, 0.}

DateString[{2012, 10, 8, 7}, TimeZone -> 0]
"Mon 8 Oct 2012 12:00:00"

To base these calculations off a different time zone, one should simply have to change the value of $TimeZone, either temporarily, using Block, or globally, by unprotecting and changing $TimeZone. But neither of these approaches works as expected. Both behave as if the machine's time zone, and not $TimeZone is being used to determine the local time zone:

Block[{$TimeZone = 0}, DateList[{2012, 10, 8, 7}, TimeZone -> 0]]
{2012, 10, 8, 12, 0, 0.}

Block[{$TimeZone = 0}, DateString[{2012, 10, 8, 7}, TimeZone -> 0]]
"Mon 8 Oct 2012 12:00:00"

(* Machine time zone is UTC-5 throughout; $TimeZone = 0 until changed below *)
Unprotect[$TimeZone]; $TimeZone = 0;

DateList[{2012, 10, 8, 7}, TimeZone -> 0]
{2012, 10, 8, 12, 0, 0.}

DateString[{2012, 10, 8, 7}, TimeZone -> 0]
"Mon 8 Oct 2012 12:00:00"

And even weirder:

DateList[{2012, 10, 8, 7}]
{2012, 10, 8, 7, 0, 0.}

DateList[{2012, 10, 8, 7}, TimeZone -> $TimeZone]
{2012, 10, 8, 12, 0, 0.}

And yet more timey-wimey:

AstronomicalData["Sun", {"RightAscension", {2012, 10, 8, 7}}]
12.958492

AstronomicalData["Sun", {"RightAscension", {2012, 10, 8, 7}}, TimeZone -> 0]
12.958492

AstronomicalData["Sun", {"RightAscension", {2012, 10, 8, 7}}, TimeZone -> $TimeZone]
12.958492

which all seems okay until¹

Unprotect[$TimeZone]; $TimeZone = -5

AstronomicalData["Sun", {"RightAscension", {2012, 10, 8, 7}}]
12.958492

AstronomicalData["Sun", {"RightAscension", {2012, 10, 8, 7}}, TimeZone -> 0]
12.945765

AstronomicalData["Sun", {"RightAscension", {2012, 10, 8, 7}}, TimeZone -> $TimeZone]
12.958492

The second set of AstronomicalData calculations (with $TimeZone = -5, the machine time zone) is at least consistent with what DateList and DateString seem to be doing: input is interpreted as being in the machine time zone (regardless of $TimeZone), unless a different time zone is specified using TimeZone. But that seems to be precisely the problem: if the time zone specified with TimeZone matches $TimeZone, it's ignored, even if the machine time zone is different. So in the first set of AstronomicalData calculations (with $TimeZone = 0), when TimeZone is '0' or $TimeZone, the option is just ignored, and AstronomicalData reports data for the default, machine time zone instead.

If $TimeZone is merely the default setting for TimeZone options and not the default time zones for executing time zone dependent functions, how do I get date and time functions to work in another time zone by default (without changing the machine's clock)?

---

^{1. I know the Sun's RA isn't the best value to use to spot differences over hour time scales, but since I can't get a straight answer from Wolfram (after 2 weeks of trying) about whether Mathematica ephemerides incorporate parallax or atmospheric effects, I wanted to be sure I had used something that would be immune to those effects when viewed from two different locations, since I don't specify those in the example.}

Answer 1

You are not using the functions correctly

In[9]:= DateString[]
Out[9]= "Tue 30 Apr 2013 09:37:20"

while

In[10]:= DateString[TimeZone -> 0]
Out[10]= "Tue 30 Apr 2013 07:37:26"

and

In[33]:= AstronomicalData["Sun", {"NextRiseTime", {2012, 10, 8, 6, 0, 
   0}}, TimeZone -> 1]
Out[33]= {2012, 10, 8, 6, 35, 31.7528}

In[34]:= AstronomicalData["Sun", {"NextRiseTime", {2012, 10, 8, 6, 0, 
   0}}, TimeZone -> 4]
Out[34]= {2012, 10, 8, 9, 35, 31.722}

work as expected.