Fractional Seconds in Date/Time Values

[oleibman]

This is a replacement for PR #2404 which has been open for almost 2 years, and which I will close now. As submitted, it broke many unit tests, and no attempt was made to fix those and add others. However, while reviewing it, I found that, among all the tests which it accidentally broke, there were tests which it "broke" (in ExplicitDateTest) which were actually wrong in the first place. That seemed a good enough reason to investigate further.

The original PR suggested the change was needed because "there is now support for microseconds when reading Datetime cells". I'm not sure that's true. Time of day is stored as a fraction of a day, and nothing prevents microseconds from being part of that fraction. It is true that Excel does not allow you to format a date/time cell to display more than 3 decimal positions for the seconds value, even if the value turns out to be accurate to the microsecond, and that has not changed. It is also true that Php supports microsecond accuracy in its DateTime objects, and it behooves PhpSpreadsheet to accommodate that.

PhpSpreadsheet, like Excel, nominally supported the use of one, two, or three decimals when displaying seconds. However, it did not do it correctly, and there had been no tests of this using a value where the decimals were anything other than 0. One existing test, in NumberFormatDates, was wrong. It is fixed and new tests added.

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.