[C++] Revert changes to qualify duration in vendored date code

[asfimport]

The changes where made to get PR #3644 compiling without using "duration" type constructor. Vendored code changes are bad ...

Reporter: Micah Kornfield / @emkornfield
Assignee: Micah Kornfield / @emkornfield

PRs and other links:

• GitHub Pull Request #4324

_{Note: This issue was originally created as ARROW-5346. Please see the migration documentation for further details.}

[asfimport]

Pindikura Ravindra / @pravindra:
Issue resolved by pull request 4324
#4324