diff --git a/khard/contacts.py b/khard/contacts.py
index 9b20aef0..51252686 100644
--- a/khard/contacts.py
+++ b/khard/contacts.py
@@ -33,7 +33,7 @@
 from . import address_book  # pylint: disable=unused-import # for type checking
 from . import helpers
 from .helpers.typing import (Date, PostAddress, StrList, convert_to_vcard,
-    list_to_string, string_to_date, string_to_list)
+    list_to_string, string_to_date, string_to_list, DEFAULT_YEAR)
 from .query import AnyQuery, Query
 
 
@@ -450,7 +450,7 @@ def _prepare_birthday_value(self, date: Date) -> tuple[Optional[str],
                 return date.strip(), True
             return None, False
         tz = date.tzname()
-        if date.year == 1900 and date.month != 0 and date.day != 0 \
+        if date.year == DEFAULT_YEAR and date.month != 0 and date.day != 0 \
                 and date.hour == 0 and date.minute == 0 and date.second == 0 \
                 and self.version == "4.0":
             fmt = '--%m%d'
@@ -977,7 +977,7 @@ def _format_date_object(date: Optional[Date], localize: bool) -> str:
             return ""
         if isinstance(date, str):
             return date
-        if date.year == 1900 and date.month != 0 and date.day != 0 \
+        if date.year == DEFAULT_YEAR and date.month != 0 and date.day != 0 \
                 and date.hour == 0 and date.minute == 0 and date.second == 0:
             return date.strftime("--%m-%d")
         tz = date.tzname()
@@ -1075,8 +1075,8 @@ def _set_date(self, target: str, key: str, data: dict) -> None:
                              "with vcard version 4.0.")
         if re.match(r"^--\d\d-?\d\d$", new) and self.version != "4.0":
             raise ValueError(
-                f"{key} format --mm-dd and --mmdd only usable with "
-                "vcard version 4.0. You may use 1900 as placeholder, if "
+                f"{key} format --mm-dd and --mmdd only usable with vcard "
+                f"version 4.0. You may use {DEFAULT_YEAR} as placeholder, if "
                 "the year is unknown.")
         try:
             v2 = string_to_date(new)
diff --git a/khard/helpers/__init__.py b/khard/helpers/__init__.py
index 303c8059..bc31a21b 100644
--- a/khard/helpers/__init__.py
+++ b/khard/helpers/__init__.py
@@ -7,7 +7,7 @@
 from typing import Any, Optional, Sequence, Union
 
 from ruamel.yaml.scalarstring import LiteralScalarString
-from .typing import list_to_string, PostAddress
+from .typing import list_to_string, PostAddress, DEFAULT_YEAR
 
 
 YamlPostAddresses = dict[str, Union[list[dict[str, Any]], dict[str, Any]]]
@@ -164,7 +164,7 @@ def yaml_anniversary(anniversary: Union[str, datetime, None],
         return None
 
     if isinstance(anniversary, datetime):
-        if (version == "4.0" and anniversary.year == 1900
+        if (version == "4.0" and anniversary.year == DEFAULT_YEAR
                              and anniversary.month != 0
                              and anniversary.day != 0
                              and anniversary.hour == 0
diff --git a/khard/helpers/typing.py b/khard/helpers/typing.py
index 7e0e363d..487407bc 100644
--- a/khard/helpers/typing.py
+++ b/khard/helpers/typing.py
@@ -10,6 +10,14 @@
 PostAddress = dict[str, str]
 
 
+# A default year for datetimes without one.  Python does not support datetime
+# objects without a year from 3.15 onwards but vCard does.  We add a default
+# year to the python object and detect it again when formatting the object.
+# 1900 was the internally used default year of python's datetime object when it
+# still did support instances without a year (before python 3.5).
+DEFAULT_YEAR = 1900
+
+
 @overload
 def convert_to_vcard(name: str, value: StrList, constraint: type[str]) -> str: ...
 @overload
@@ -67,20 +75,44 @@ def string_to_date(string: str) -> datetime:
     :param string: the date string to parse
     :returns: the parsed datetime object
     """
-    # try date formats --mmdd, --mm-dd, yyyymmdd, yyyy-mm-dd and datetime
-    # formats yyyymmddThhmmss, yyyy-mm-ddThh:mm:ss, yyyymmddThhmmssZ,
-    # yyyy-mm-ddThh:mm:ssZ.
-    for fmt in ("--%m%d", "--%m-%d", "%Y%m%d", "%Y-%m-%d", "%Y%m%dT%H%M%S",
-                "%Y-%m-%dT%H:%M:%S", "%Y%m%dT%H%M%SZ", "%Y-%m-%dT%H:%M:%SZ"):
+
+    # Attempt to parse the string as any of the date and time formats supported
+    # by Khard, as defined by the vCard and ISO 8601:2000 specifications.
+    # Strings which define a day-of-month but not a year are ambiguous, require
+    # special handling, and will be unsupported in Python >= 3.15. (They were
+    # already removed in ISO 8601:2004, but remain a part of vCard.)
+
+    # Ambiguous cases of a date with no year (--%m%d and --%m-%d).
+    try:
+        if string.startswith("--"):
+            tmp = str(DEFAULT_YEAR) + string[2:]
+            if "-" in string[2:]:
+                return datetime.strptime(tmp, "%Y%m-%d")
+            else:
+                return datetime.strptime(tmp, "%Y%m%d")
+    except ValueError:
+        pass
+
+    # Fully qualified date and time formats.
+    for fmt in (
+        "%Y%m%d",
+        "%Y-%m-%d",
+        "%Y%m%dT%H%M%S",
+        "%Y-%m-%dT%H:%M:%S",
+        "%Y%m%dT%H%M%SZ",
+        "%Y-%m-%dT%H:%M:%SZ",
+    ):
         try:
             return datetime.strptime(string, fmt)
         except ValueError:
-            continue  # with the next format
-    # try datetime formats yyyymmddThhmmsstz and yyyy-mm-ddThh:mm:sstz where tz
-    # may look like -06:00.
+            continue
+
+    # Timezone formats which may contain a problematic colon.
     for fmt in ("%Y%m%dT%H%M%S%z", "%Y-%m-%dT%H:%M:%S%z"):
         try:
-            return datetime.strptime(''.join(string.rsplit(":", 1)), fmt)
+            return datetime.strptime("".join(string.rsplit(":", 1)), fmt)
         except ValueError:
-            continue  # with the next format
+            continue
+
+    # All formats tried. Date cannot be parsed.
     raise ValueError