date arithmetic returns date instead of number

[MartijnR]

These (new) tests fail:

 it('example 4', () => {
     assertStringValue('"2021-11-30" + 1', '18962'); // returns '2021-11-01' instead
});

it('example 5', () => {
     assertStringValue('"2021-11-30" - "2021-11-29"', '1'); // returns '1970-01-02' instead
});

Both ODK Collect and the old Enketo evaluator return a number (as string), following the XPath rule that the presence of the operator (+) causes both left and right operands to be converted to a number.

I suspect, I may be responsible for this change, so sorry about that. I am hoping the fix may actually simplify the code.

Note, that if behind the scenes, the number value is not an integer (but a decimal), I think it might be pragmatic to consider that a separate (timezone) issue in either Collect or Enketo (and leave it for later).

[MartijnR]

Removing this code makes these tests pass (though decimal 18962.291666666668 is the result for the first).

There is one karma test that will start failing which is:

#date() date calculations should convert today() > ('2012-01-01' + 10) to true

Adding another test like 'date("2100-01-02") > 1' (which should be true but is false), reveals that comparisons with numbers and dates do not work currently in master. That is probably the cause of that.

Though some other mocha tests are failing. Tbc (probably requires changing expected results to numbers).

[MartijnR]

I added the above tests to the fix/dates-141 branch (though integer results can be decimals in reality as mentioned above).