dateTime.val (or `_dateVal`) fails for dates before 1900

In the C++ version of MiniScript, the `dateTime.val` function (from the standard library, which simply calls the `_dateVal` intrinsic function) returns -946710001 for any date before the year 1900.

Example:
```
> dateTime.val("1900-01-01") 
-3155673600
> dateTime.val("1899-01-01")
-946710001
> dateTime.val("1898-01-01") 
-946710001
```
