Fix support for negative timestamps

[alfaproject]

In the date conversion methods negative dates are not support and that doesn't allow to have people that were born before 1970 in our database.

Example: https://github.com/ironSource/parquetjs/blob/master/lib/types.js#L300

We created a patch to remove the negative check but it would be good to have it done here.

[dobesv]

Did you submit that change as a pull request?