date_time library supports only the current dst rule

[ljj116]

As one could expect, from the layout of the csv file, the date_time library only supports the current daylight saving time (DST) rule.  This issue will become more apparent once the csv file is updated to support the new DST rules for the US.  Dates from 2007 forward will be correct, but differentials calculated with years prior to 2007 may be incorrect.

For instance if you do a "seconds since epoch" conversion on a date between March 12, 2006 and April 2, 2006 (gap between old rules and new rules), the result will be skewed by 3600 seconds (one hour).  This will be because the library thinks that DST was in effect during this time (new rules), but it was not (old rules).

[Katie Chan]

Duplicate of #605.