diff --git a/src/api/main.py b/src/api/main.py
index 4e861c1..0d96293 100644
--- a/src/api/main.py
+++ b/src/api/main.py
@@ -24,6 +24,7 @@ from api.auth import get_current_user, resolve_account_for_user
 from api.schemas import ContactListResponse, ContactOut, SyncRunOut
 from config import Config
 from mailer import build_message, fetch_birthdays_for_date, send_message
+from utils import fmt_birthday_age, fmt_birthday_short
 
 logging.basicConfig(level=Config.LOG_LEVEL, format="%(asctime)s [%(levelname)s] %(message)s")
 logger = logging.getLogger("api")
@@ -33,6 +34,8 @@ app.add_middleware(SessionMiddleware, secret_key=secrets.token_hex(32), session_
 app.mount("/static", StaticFiles(directory="api/static"), name="static")
 templates = Jinja2Templates(directory="api/templates")
 templates.env.filters["urlquote"] = lambda s: quote_plus(s or "")
+templates.env.filters["fmt_birthday"] = fmt_birthday_short
+templates.env.filters["fmt_age"] = fmt_birthday_age
 
 
 def _fmt_ts(dt) -> str | None:
diff --git a/src/api/templates/contact.html b/src/api/templates/contact.html
index 578df63..da6646f 100644
--- a/src/api/templates/contact.html
+++ b/src/api/templates/contact.html
@@ -332,7 +332,7 @@
                 {% if contact.birthday %}
                 <div class="section">
                     <div class="birthday-highlight">
-                        🎂 {{ contact.birthday }}
+                        🎂 {{ contact.birthday|fmt_birthday }}
                     </div>
                 </div>
                 {% endif %}
diff --git a/src/api/templates/dashboard.html b/src/api/templates/dashboard.html
index 9240f13..c1061a3 100644
--- a/src/api/templates/dashboard.html
+++ b/src/api/templates/dashboard.html
@@ -367,7 +367,7 @@
                             <div class="birthday-organization">{{ b.organization }}</div>
                             {% endif %}
                             <div class="birthday-date">
-                                {{ b.birthday.strftime('%d.%m.') }}{% if b.birthday.year and b.birthday.year < current_year %} · {{ current_year - b.birthday.year }} Jahre{% endif %}
+                                {{ b.birthday|fmt_birthday }}{% if b.birthday|fmt_age %} · {{ b.birthday|fmt_age(current_year) }} Jahre{% endif %}
                             </div>
                         </div>
                         {% if b.photo_url %}
diff --git a/src/api/templates/index.html b/src/api/templates/index.html
index 2e2ad1c..4c89376 100644
--- a/src/api/templates/index.html
+++ b/src/api/templates/index.html
@@ -198,7 +198,7 @@
                             <div class="contact-organization">{{ c.organization }}</div>
                             {% endif %}
                             {% if c.birthday %}
-                            <div class="contact-birthday">{{ c.birthday }}</div>
+                            <div class="contact-birthday">{{ c.birthday|fmt_birthday }}</div>
                             {% endif %}
                         </div>
                         <div style="display:flex;flex-direction:column;align-items:center;gap:0.3rem">
diff --git a/src/mailer.py b/src/mailer.py
index b79181f..11044b8 100644
--- a/src/mailer.py
+++ b/src/mailer.py
@@ -13,6 +13,7 @@ from email.message import EmailMessage
 
 import db
 from config import Config
+from utils import fmt_birthday_age, fmt_birthday_short
 
 logging.basicConfig(level=Config.LOG_LEVEL, format="%(asctime)s [%(levelname)s] %(message)s")
 logger = logging.getLogger("mailer")
@@ -68,9 +69,10 @@ def build_message(birthdays: list[dict], target_date: date | None = None) -> Ema
 
     plain_lines = [f"Heutige Geburtstage ({today.isoformat()}):", ""]
     for b in birthdays:
-        age = today.year - b["birthday"].year
-        date_str = b["birthday"].strftime("%d.%m.")
-        line = f"- {b['full_name']}  · {date_str} · {age} Jahre"
+        date_str = fmt_birthday_short(b["birthday"])
+        age = fmt_birthday_age(b["birthday"], today)
+        age_str = f" · {age} Jahre" if age is not None else ""
+        line = f"- {b['full_name']}  · {date_str}{age_str}"
         if b.get("organization"):
             line += f"  ({b['organization']})"
         if Config.WEB_URL:
@@ -82,8 +84,9 @@ def build_message(birthdays: list[dict], target_date: date | None = None) -> Ema
 
     cards_html = ""
     for b in birthdays:
-        age = today.year - b["birthday"].year
-        date_str = b["birthday"].strftime("%d.%m.")
+        date_str = fmt_birthday_short(b["birthday"])
+        age = fmt_birthday_age(b["birthday"], today)
+        age_str = f" · {age} Jahre" if age is not None else ""
         contact_link = f"{Config.WEB_URL.rstrip('/')}/contacts/{b['id']}" if Config.WEB_URL else ""
         name_html = f'<a href="{contact_link}" style="color:#222;text-decoration:none;">{b["full_name"]}</a>' if contact_link else b["full_name"]
         is_today = b["birthday"].month == today.month and b["birthday"].day == today.day
@@ -99,7 +102,7 @@ def build_message(birthdays: list[dict], target_date: date | None = None) -> Ema
               <div>
                 <div style="font-size:18px;font-weight:600;color:#222;">{name_html}</div>
                 {org_html}
-                <div style="font-size:14px;color:#888;margin-top:4px;">🎂 {date_str} · {age} Jahre</div>
+                <div style="font-size:14px;color:#888;margin-top:4px;">🎂 {date_str}{age_str}</div>
               </div>
               {photo_html}
             </div>
diff --git a/src/utils.py b/src/utils.py
new file mode 100644
index 0000000..0a6c2db
--- /dev/null
+++ b/src/utils.py
@@ -0,0 +1,22 @@
+# Copyright (c) 2026 Stefan Koelle (https://stefankoelle.de)
+# Licensed under the MIT License. See LICENSE file in project root for details.
+from datetime import date
+
+UNKNOWN_YEARS = frozenset({0, 1604, 1900})
+
+
+def is_unknown_year(birthday: date) -> bool:
+    return birthday.year in UNKNOWN_YEARS
+
+
+def fmt_birthday_short(birthday: date) -> str:
+    if is_unknown_year(birthday):
+        return birthday.strftime("%d.%m.")
+    return birthday.strftime("%d.%m.%Y")
+
+
+def fmt_birthday_age(birthday: date, reference) -> int | None:
+    if is_unknown_year(birthday):
+        return None
+    ref_year = reference if isinstance(reference, int) else reference.year
+    return ref_year - birthday.year