Zero-Indexing the Gregorian Calendar

Biggest complication with calendars is converting between them, or rather, knowing what was the calendar in use when a particular date is mentioned.

If I say an event happened on January 5th, 1603, you can't really know what was the date in present day calendar (proleptic Gregorian calendar) really without a bit more context.

And adding a new calendaring system, even if it reduces some ambiguities, does not help, but quite the opposite.

Of one wants an unambiguous date and time measure that covers arguably all of human history, julian day numbers (JDN) are a perfectly suitable measure. It's similar to the Unix epoch, except that it's in days and starts a bit earlier :)

Careful with calendar systems; it seems that even minor scholarship on the subject leads quickly to madness.