rlsnotes40 missed information on changes to datetime literal syntax

Author: mrotteveel

src/docs/asciidoc/en/rlsnotes/rlsnotes40/_compatibility-40.adoc

@@ -204,6 +204,51 @@ Enforcement of the appropriate usage means that attempting to combine both becom
 Existing code where usage does not break the rule remains unaffected.
 Both `'NOW'` and `CAST('NOW' AS TIMESTAMP)` continue to work as before, as well as code that correctly uses the date/time prefixes with explicit literals,  like `DATE '2019-02-20'`.
 
+[#rnfb40-compat-datetime-string]
+==== Datetime Literal Syntax Changes
+
+To avoid ambiguities with time zones, the datetime literal syntax -- also used when casting strings to datetime types -- had to be changed.
+In previous versions, any element of a datetime (e.g. year, minutes, etc.) could be separated by whitespace, `.`, `:`, `,`, `-`, and `/`.
+
+The revised syntax is:
+
+.Datetime Format Syntax
+[listing,subs="+quotes,attributes"]
+----
+<date_format> ::=
+      [__YYYY__<p>{endsb}__MM__<p>__DD__
+    | __MM__<p>__DD__[<p>{ __YYYY__ | __YY__ }]
+    | __DD__<p>__MM__[<p>{ __YYYY__ | __YY__ }]
+
+<time_format> ::= __HH__[:__mm__[:__SS__[<f>__NNNN__]]]
+
+<timestamp_format> ::= <date_format> [<space> <time_format>]
+
+<time_zone> ::=
+    { + | - }__HH__:__MM__
+  | __time zone name (e.g. Europe/Berlin)__
+
+<time_tz_format> ::= <time_format> [<space>] <time_zone>
+
+<timestamp_tz_format> ::= <timestamp_format> [<space>] <time_zone>
+
+<p> ::= whitespace | . | - | /
+<f> ::= : | .
+----
+
+[NOTE]
+====
+The choice of the separator `<p>` in a date decides whether the parser reads `MM<p>DD` or `DD<p>MM`.
+====
+
+[TIP]
+====
+Although the Firebird datetime syntax is still very flexible, to avoid ambiguity we recommend using the ISO-8601 order (year-month-day), '```-```' as the date separator, 4 digits for year, 2 digits for month, day, minute and second, and '```.```' as second.fractions separator.
+This format is also the only one defined in the SQL standard.
+
+In short, use `TIMESTAMP '2021-05-03 04:05:00.1 +02:00'`, not `TIMESTAMP '3.5.21 4:5:0:1 +2:0'`.
+====
+
 [[rnfb40-compat-sql-sequence-start-value]]
 === Starting Value of Sequences