change ms/s watershed for datetime parsing

[samuelcolvin]

Change Summary

Change the second/millisecond watershed for date/datetime parsing to 2e10

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• HISTORY.rst has been updated
  • if this is the first change since a release, please add a new section
  • include the issue number or this pull request number #<number>
  • include your github username @<whomever>

[codecov]

Codecov Report

Merging #385 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #385   +/-   ##
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  Coverage     100%   100%           
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  Files          14     14           
  Lines        2000   2000           
  Branches      375    375           
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  Hits         2000   2000