Model: Fix representation of negative dates

[gflima]

We are currently ignoring the negative sign in timestamps. Note that Python's datetime does not support negative dates!

[gflima]

Also, handle 0's:

2015-07-00T00:00:00Z

[gflima]

The problem is not only negative dates, but also dates with a year greater than 9999, e.g., stuff related to galaxies, dinosaurs, etc. I think the best approach is to get rid of datetime, and use a custom representation for dates.