6039/5937: Date_Time Assertion failures encountered operating Plots
Bug Number: 6039/5937
Release notes (y/n): Yes
For Release Nums: 7.2

The process of computing a plot's tick marks along the time axes was 
sometimes presenting dates before 1800 to Date_Time (C++ class) computations.
This resulted in unexpected negative numbers being detected in Date_Time:: 
get_yday(), failing an assertion.  This was seen in a specific test sequence 
documented with Gnats 6039, but we believe that this was also the cause of 
the Gnats 5937 errors.  See this illustration:

  http://cadswes2.colorado.edu/~philw/pub/6039/Bug6039-Plot1.png

The primary fix is in the TimeScaleEngine::buildTicks() method, in Q3GUI/
SlotPlot.cpp during an iteration through the plot's visible time range.
We now skip days before the beginning of the supported "epoch" (starting 
in the year 1800).

Also, the ASSERTION in method Date_Time::get_yday() has been replaced with 
an INTERNAL WARNING providing more information about the condition.
PLEASE DO report any occurrence of such internal Date_Time errors.
We don't expect those to happen.

Also, the cursor coordinates shown on a plot's status bar were showing bogus 
date/times for the time axis if the cursor was to the left of the year 1800.
This has also been fixed.  See the following two illustrations:

  http://cadswes2.colorado.edu/~philw/pub/6039/BadCursorDateTimeStatusBefore1800.png
  http://cadswes2.colorado.edu/~philw/pub/6039/FixCursorDateTimeStatusBefore1800.png

---