diff --git a/backend/app/routers/auth.py b/backend/app/routers/auth.py
index 380e29c..28f5848 100644
--- a/backend/app/routers/auth.py
+++ b/backend/app/routers/auth.py
@@ -1,6 +1,6 @@
 """Authentication router - login, register, token refresh."""
 
-from datetime import datetime, timedelta, timezone
+from datetime import date, datetime, timedelta, timezone
 
 from fastapi import APIRouter, Depends, HTTPException, status
 from fastapi.security import OAuth2PasswordRequestForm
@@ -77,9 +77,19 @@ def hash_password(password: str) -> str:
     return pwd_context.hash(password)
 
 
-def calculate_age(birth_date: datetime) -> int:
-    """Calculate age in years from a birth date."""
-    today = datetime.now(timezone.utc).date()
+def calculate_age(birth_date: date, today: date | None = None) -> int:
+    """Calculate age in years from a birth date.
+
+    Args:
+        birth_date: The person's date of birth.
+        today: The reference date. Defaults to today's date (UTC).
+
+    Returns:
+        Age in whole years as of the reference date.
+    """
+    if today is None:
+        today = datetime.now(timezone.utc).date()
+
     age = today.year - birth_date.year
     # Adjust if birthday hasn't occurred yet this year
     if (today.month, today.day) < (birth_date.month, birth_date.day):
diff --git a/backend/tests/test_auth_age_gate.py b/backend/tests/test_auth_age_gate.py
index 389eaf5..73a9f0a 100644
--- a/backend/tests/test_auth_age_gate.py
+++ b/backend/tests/test_auth_age_gate.py
@@ -1,7 +1,6 @@
 """Tests for the 18+ age gate on user registration."""
 
-from datetime import date, timedelta, datetime
-from unittest.mock import patch
+from datetime import date, timedelta
 
 import pytest
 
@@ -13,52 +12,83 @@ class TestCalculateAge:
 
     def test_exactly_18(self):
         """A person born exactly 18 years ago today is 18."""
-        today = datetime.now().date()
+        today = date(2025, 6, 15)
         dob = today.replace(year=today.year - 18)
-        assert calculate_age(dob) == 18
+        assert calculate_age(dob, today=today) == 18
 
     def test_just_turned_18(self):
         """A person who turned 18 yesterday is 18."""
-        today = datetime.now().date()
+        today = date(2025, 6, 15)
         dob = today.replace(year=today.year - 18) - timedelta(days=1)
-        assert calculate_age(dob) == 18
+        assert calculate_age(dob, today=today) == 18
 
     def test_one_day_short_of_18(self):
         """A person born one day after their 18th birthday is 17."""
-        today = datetime.now().date()
+        today = date(2025, 6, 15)
         dob = today.replace(year=today.year - 18) + timedelta(days=1)
-        assert calculate_age(dob) == 17
+        assert calculate_age(dob, today=today) == 17
 
     def test_17_years_old(self):
-        """A person who is 17 years and 364 days old is 17."""
-        today = datetime.now().date()
-        dob = today.replace(year=today.year - 17) + timedelta(days=1)
-        assert calculate_age(dob) == 17
+        """A person who is 17 years old today is 17."""
+        today = date(2025, 6, 15)
+        dob = date(2007, 6, 15)
+        assert calculate_age(dob, today=today) == 17
 
     def test_65_years_old(self):
         """A person who is 65 is correctly calculated."""
-        today = datetime.now().date()
+        today = date(2025, 6, 15)
         dob = today.replace(year=today.year - 65)
-        assert calculate_age(dob) == 65
+        assert calculate_age(dob, today=today) == 65
 
     def test_newborn(self):
         """A newborn is 0."""
-        today = datetime.now().date()
+        today = date(2025, 6, 15)
         dob = today
-        assert calculate_age(dob) == 0
+        assert calculate_age(dob, today=today) == 0
 
     def test_birthday_not_yet_this_year(self):
         """Person born Dec 31, 2000 — on Jan 1, 2025 they are still 24."""
         dob = date(2000, 12, 31)
-        with patch("app.routers.auth.datetime") as mock_dt:
-            mock_dt.now.return_value = datetime(2025, 1, 1)
-            mock_dt.now.return_value.date.return_value = date(2025, 1, 1)
-            assert calculate_age(dob) == 24
+        assert calculate_age(dob, today=date(2025, 1, 1)) == 24
 
     def test_birthday_today(self):
         """Person born Dec 31, 2000 — on Dec 31, 2024 they turn 24."""
         dob = date(2000, 12, 31)
-        with patch("app.routers.auth.datetime") as mock_dt:
-            mock_dt.now.return_value = datetime(2024, 12, 31)
-            mock_dt.now.return_value.date.return_value = date(2024, 12, 31)
-            assert calculate_age(dob) == 24
+        assert calculate_age(dob, today=date(2024, 12, 31)) == 24
+
+
+class TestRegisterRequestDateValidation:
+    """Tests for the RegisterRequest date_of_birth field validator."""
+
+    @pytest.mark.parametrize(
+        "dob",
+        [
+            "2000-01-01",
+            "1990-12-31",
+            "2004-02-29",  # leap day
+        ],
+    )
+    def test_valid_dates(self, dob):
+        """Valid YYYY-MM-DD dates pass validation."""
+        from app.routers.auth import RegisterRequest
+
+        req = RegisterRequest(username="u", email="u@example.com", password="p", date_of_birth=dob)
+        assert req.date_of_birth == dob
+
+    @pytest.mark.parametrize(
+        "dob",
+        [
+            "01-01-2000",
+            "2000/01/01",
+            "2000-13-01",
+            "2000-00-10",
+            "not-a-date",
+            "",
+        ],
+    )
+    def test_invalid_dates_rejected(self, dob):
+        """Invalid dates are rejected by the validator."""
+        from app.routers.auth import RegisterRequest
+
+        with pytest.raises(ValueError):
+            RegisterRequest(username="u", email="u@example.com", password="p", date_of_birth=dob)