Fix#4696. Fix#917 (which had gone stale but is now reopened). Fix#916. People were receiving unexpected exceptions calling `Date::isDateTime($cell)`. 916 and 917 had been mostly resolved long ago by adding a numeric check on the contents of $cell. However, it turns out that the use of values with very large absolute values can cause Php DateTime to throw an exception, which is the problem with 4696, when it calls `Date::toExcelDateTimeObject` for a cell with a large integer value.
`Date::toExcelDateTimeObject` is coded to return a Php DateTime object. I don't see any reasonable non-breaking fix for that. People can always add try/catch if they are worried about that. However, `Date::isDateTime` can add an additional check and return `false` if this situation occurs. All 3 of the issues resolved by this PR mention isDateTime in their discussion; they are all fixed without additional coding changes by this.
An additional change is needed for `Cell::getFormattedValue` to avoid this problem. This is fixed in `Style::NumberFormatter::DateFormatter::format`.
oleibman