Explicitly disallow year 0

Year "0" doesn't exist; it goes from 1 BC to 1 AD.

RFC 3339 is somewhat unclear if this should be allowed, at least in my
reading of it:

> This document defines a [..] representation of dates and times using
> the Gregorian calendar.

"Gregorian calendar" has no "year zero", so it should be forbidden. But
also:

> All dates and times are assumed to be in the "current era",
> somewhere between 0000AD and 9999AD.

So meh.

Practically speaking, supporting this across the board is rather tricky.
Python's datetime has no way to represent this (other than None, maybe?
Ugh.), PostgreSQL doesn't support it, Go's time.Time behaves oddly (e.g.
IsZero() is false, which is rather surprising), etc.

ISO 8601 defines year 0 as "1 BC", which is even worse since most
datetime implementations don't really do BC dates.

Just forbidding it is by far the easiest for everyone; for
implementations with a datetime that supports it, it's just a single
`if`, and for e.g. Python nothing needs to be done.

The only potential downside is that people may have `d = 0000-01-01`. We
already broke compatibility "for sanity" by disallowing table overrides.
The alternative is making it implementation-dependent. Meh.

RFC 3339 is supposed to be a "ISO 8601, without obscure edge cases";
this seems to fit with the intended purpose.

CHANGELOG.md

@@ -12,6 +12,7 @@
 - Seconds in Date-Time and Time values are now optional.
 - Allow non-English scripts in unquoted (bare) keys
 - Clarify newline normalization in multi-line literal strings.
+- Explicitly disallow year zero.
 
 ## 1.0.0 / 2021-01-11
 

toml.abnf

@@ -177,7 +177,7 @@ false   = %x66.61.6C.73.65  ; false
 
 date-time      = offset-date-time / local-date-time / local-date / local-time
 
-date-fullyear  = 4DIGIT
+date-fullyear  = 4DIGIT  ; 0001-9999
 date-month     = 2DIGIT  ; 01-12
 date-mday      = 2DIGIT  ; 01-28, 01-29, 01-30, 01-31 based on month/year
 time-delim     = "T" / %x20 ; T, t, or space

toml.md

@@ -582,6 +582,13 @@ implementation-specific. If the value contains greater precision than the
 implementation can support, the additional precision must be truncated, not
 rounded.
 
+The first year in the Gregorian calendar is year 1 (1 BC is followed by 1 AD),
+and year 0 is not allowed:
+
+```
+odt7 = 0000-01-01 07:32:32Z  # INVALID
+```
+
 ## Local Date-Time
 
 If you omit the offset from an [RFC 3339](https://tools.ietf.org/html/rfc3339)