Fix issue with negative times in `date_time` and add tests for full coverage

[Flamefire]

When handling negative times (double value) the fractional part (i.e. nanoseconds) is subtracted from the seconds to make the nanoseconds positive. However the subtraction happened on a temporary value, not the result struct leading to off-by-one errors

Also add some more tests to fully cover date_time.cpp (which led to the discovery of the above bug), especially the date_time constructors taking a calendar and the handling of the "active" time-zone were not sufficiently tested

[codecov[bot]]

Codecov Report

Merging #198 (303e141) into develop (2cdef1a) will increase coverage by 0.32%. The diff coverage is 100.00%.

Additional details and impacted files

[](https://app.codecov.io/gh/boostorg/locale/pull/198?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) ```diff @@ Coverage Diff @@ ## develop #198 +/- ## =========================================== + Coverage 94.93% 95.26% +0.32% =========================================== Files 114 114 Lines 9917 10001 +84 =========================================== + Hits 9415 9527 +112 + Misses 502 474 -28 ``` | [Files](https://app.codecov.io/gh/boostorg/locale/pull/198?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) | Coverage Δ | | |---|---|---| | [include/boost/locale/date\_time.hpp](https://app.codecov.io/gh/boostorg/locale/pull/198?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-aW5jbHVkZS9ib29zdC9sb2NhbGUvZGF0ZV90aW1lLmhwcA==) | `95.03% <100.00%> (+0.10%)` | :arrow_up: | | [include/boost/locale/hold\_ptr.hpp](https://app.codecov.io/gh/boostorg/locale/pull/198?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-aW5jbHVkZS9ib29zdC9sb2NhbGUvaG9sZF9wdHIuaHBw) | `94.44% <100.00%> (-1.02%)` | :arrow_down: | | [src/boost/locale/shared/date\_time.cpp](https://app.codecov.io/gh/boostorg/locale/pull/198?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-c3JjL2Jvb3N0L2xvY2FsZS9zaGFyZWQvZGF0ZV90aW1lLmNwcA==) | `100.00% <100.00%> (+12.90%)` | :arrow_up: | | [test/test\_date\_time.cpp](https://app.codecov.io/gh/boostorg/locale/pull/198?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-dGVzdC90ZXN0X2RhdGVfdGltZS5jcHA=) | `100.00% <100.00%> (ø)` | | ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/boostorg/locale/pull/198?src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). > **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) > `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` > Powered by [Codecov](https://app.codecov.io/gh/boostorg/locale/pull/198?src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). Last update [2cdef1a...303e141](https://app.codecov.io/gh/boostorg/locale/pull/198?src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg).