Make dates with numerical offsets get parsed correctly

[DeeUnderscore]

Currently, if you specify a date like #2020-01-01 05:00:00 +05:00#, instead of getting 5am in the UTC+5 timezone, you will get UTC 5am converted to the UTC+5 timezone (ie, 10am). You can test this by comparing dates that are the same and should return a time difference of 0:

> #2020-01-01 05:00:00 +05:00# - #2020-01-01 00:00:00 +00:00#
5 hour, 0 second (time)

This PR makes dates specified with offsets get parsed as local time at that offset, which is the probably what's expected. There is some code duplication here that could perhaps be avoided with wider refactoring, but it achieves the correct behavior.

[codecov[bot]]

Codecov Report

Merging #51 into master will decrease coverage by 0.04%. The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
- Coverage   82.88%   82.83%   -0.05%     
==========================================
  Files          20       20              
  Lines        4854     4858       +4     
==========================================
+ Hits         4023     4024       +1     
- Misses        831      834       +3

Impacted Files	Coverage Δ
tests/query.rs	98.29% <100%> (+0.02%)	:arrow_up:
src/date.rs	95.8% <50%> (-0.41%)	:arrow_down:
src/substance.rs	47.2% <0%> (-0.27%)	:arrow_down:

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[tiffany352]

Thanks!