I ran the test suite using 32-bit PHP. There were 2 places where changes
were needed due to 32-bit timestamps.

Reader\Xml.php was using strtotime as an intermediate step in converting
a string timestamp to an Excel timestamp. The XML file type stores pure timestamps
(i.e. no date portion) as, e.g., 1899-12-31T02:30:00.000, and that value
causes an error using strtotime on a 32-bit system. However, it is sufficient
to use that value in a DateTime constructor, and that will work for 32- and 64-bit.

There was no test for that particular cell, so I added one to the XML read test.
And that's when I discovered the getFormattedValue bug. The cell's format
is hh":"mm":"ss. The quotes around the colons are disrupting the formatting.
PhpSpreadsheet formats the cell by converting the Excel format
to a Php Date format, in this case H\:m\:s.
That's a problem,
since Excel thinks 'm' means minutes, but PHP thinks it means months.
This is not a problem when the colon is not quoted; there are ample tests for that.
I added my best guess as to how to recognize this situation,
changing \:m to :i. The XML read test
now succeeds, and no other tests were broken by this change.

Test Shared\DateTest had one test where the expected result of converting to a
Unix timestamp exceeds 2**32. Since a Unix timestamp is strictly an int,
that test fails on a 32-bit system. In the discussion regarding recently merged
PR #1870, it was felt that the user base might still be using the functions
that convert to and from a timestamp. So, we should not drop this test, but,
since it cannot succeed on a 32-bit system, I changed it to be skipped
whenever the expected result exceeded PHP_INT_MAX. There are 3 "toTimestamp"
functions within that test. Only one of these had been affected, but I thought
it was a good idea to add additional tests to the others to demonstrate this
condition.

In the course of testing, I also discovered some 32-bit problems with
bitwise and base-conversion functions. I am preparing separate PRs to
deal with those.

This is:

- [x] a bugfix
- [ ] a new feature

Checklist:

• Changes are covered by unit tests
• 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
• Documentation is updated as necessary

Why this change is needed?

Bugfix, described above.