diff --git a/CHANGELOG.md b/CHANGELOG.md
index 14d3cebc..b7e346e9 100644
--- a/CHANGELOG.md
+++ b/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
 
diff --git a/toml.abnf b/toml.abnf
index 0446f8b6..dedc4ee4 100644
--- a/toml.abnf
+++ b/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
diff --git a/toml.md b/toml.md
index 08cc9f67..9b5b3dfd 100644
--- a/toml.md
+++ b/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)