diff --git a/atproto/syntax/testdata/datetime_parse_invalid.txt b/atproto/syntax/testdata/datetime_parse_invalid.txt
index 3672453a..3b3031c6 100644
--- a/atproto/syntax/testdata/datetime_parse_invalid.txt
+++ b/atproto/syntax/testdata/datetime_parse_invalid.txt
@@ -5,3 +5,6 @@
 1985-04-12T25:20:50.123Z
 1985-04-12T23:99:50.123Z
 1985-04-12T23:20:61.123Z
+
+# ISO-8601, but normalizes to a negative time
+0000-01-01T00:00:00+01:00
diff --git a/atproto/syntax/testdata/datetime_syntax_invalid.txt b/atproto/syntax/testdata/datetime_syntax_invalid.txt
index a686f717..9db91eb6 100644
--- a/atproto/syntax/testdata/datetime_syntax_invalid.txt
+++ b/atproto/syntax/testdata/datetime_syntax_invalid.txt
@@ -62,3 +62,5 @@
 1985-04-12T23:20:50.123+00
 1985-04-12T23:20:50.123+
 1985-04-12T23:20:50.123-
+
+-000001-12-31T23:00:00.000Z
diff --git a/atproto/syntax/testdata/datetime_syntax_valid.txt b/atproto/syntax/testdata/datetime_syntax_valid.txt
index 79ed5cab..18c3a425 100644
--- a/atproto/syntax/testdata/datetime_syntax_valid.txt
+++ b/atproto/syntax/testdata/datetime_syntax_valid.txt
@@ -32,3 +32,11 @@
 1985-04-12T23:20:50.1234567890Z
 1985-04-12T23:20:50.12345678901Z
 1985-04-12T23:20:50.123456789012Z
+
+# extreme but currently allowed
+0000-01-01T00:00:00.000Z
+0001-01-01T00:00:00.000Z
+0010-12-31T23:00:00.000Z
+1000-12-31T23:00:00.000Z
+1900-12-31T23:00:00.000Z
+3001-12-31T23:00:00.000Z