Add RFC7231 (HTTP Date) format

Ref: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Date
Examples:
- https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Expires
- https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Last-Modified
- https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Retry-After

Author: StephenBrown2

docs/docs/string_formatting.md

@@ -56,6 +56,9 @@ The following are methods to display a `DateTime` instance as a common format:
 >>> dt.to_cookie_string()
 'Thursday, 25-Dec-1975 14:15:16 EST'
 
+>>> dt.to_http_date_string()
+'Thu, 25 Dec 1975 19:15:16 GMT'
+
 >>> dt.to_iso8601_string()
 '1975-12-25T14:15:16-0500'
 

pendulum/constants.py

@@ -30,6 +30,7 @@
 RFC1036 = "ddd, DD MMM YY HH:mm:ss ZZ"
 RFC1123 = "ddd, DD MMM YYYY HH:mm:ss ZZ"
 RFC2822 = "ddd, DD MMM YYYY HH:mm:ss ZZ"
+RFC7231 = "ddd, DD MMM YYYY HH:mm:ss zz"
 RFC3339 = ISO8601
 RFC3339_EXTENDED = ISO8601_EXTENDED
 RSS = "ddd, DD MMM YYYY HH:mm:ss ZZ"

pendulum/datetime.py

@@ -23,6 +23,7 @@
 from .constants import RFC1036
 from .constants import RFC1123
 from .constants import RFC2822
+from .constants import RFC7231
 from .constants import RSS
 from .constants import SATURDAY
 from .constants import SECONDS_PER_DAY
@@ -56,6 +57,7 @@ class DateTime(datetime.datetime, Date):
         "rfc1123": RFC1123,
         "rfc2822": RFC2822,
         "rfc3339": lambda dt: dt.isoformat(),
+        "rfc7231": RFC7231,
         "rss": RSS,
         "w3c": W3C,
     }
@@ -280,6 +282,14 @@ def to_cookie_string(self) -> str:
         """
         return self._to_string("cookie", locale="en")
 
+    def to_http_date_string(self) -> str:
+        """
+        Format the instance as HTTP Date (RFC 7231).
+
+        HTTP dates are always expressed in GMT, never in local time.
+        """
+        return self.in_timezone("GMT")._to_string("rfc7231", locale="en")
+
     def to_iso8601_string(self) -> str:
         """
         Format the instance as ISO 8601.

tests/datetime/test_strings.py

@@ -38,6 +38,11 @@ def test_to_cookie_string():
     assert d.to_cookie_string() == "Thursday, 25-Dec-1975 14:15:16 EST"
 
 
+def test_to_http_date_string():
+    d = pendulum.datetime(1975, 12, 25, 14, 15, 16, tz="local")
+    assert d.to_http_date_string() == "Thu, 25 Dec 1975 19:15:16 GMT"
+
+
 def test_to_iso8601_string():
     d = pendulum.datetime(1975, 12, 25, 14, 15, 16, tz="local")
     assert d.to_iso8601_string() == "1975-12-25T14:15:16-05:00"