diff --git a/mod/hovercard.php b/mod/hovercard.php
index f5ad3ef02..616064276 100644
--- a/mod/hovercard.php
+++ b/mod/hovercard.php
@@ -107,7 +107,7 @@ function hovercard_content()
 		'about'    => $contact['about'],
 		'network'  => Strings::formatNetworkName($contact['network'], $contact['url']),
 		'tags'     => $contact['keywords'],
-		'bd'       => $contact['birthday'] <= '0001-01-01' ? '' : $contact['birthday'],
+		'bd'       => $contact['birthday'] <= DBA::NULL_DATE ? '' : $contact['birthday'],
 		'account_type' => Contact::getAccountType($contact),
 		'actions'  => $actions,
 	];
diff --git a/mod/profiles.php b/mod/profiles.php
index 459a1c5e7..fe3b36231 100644
--- a/mod/profiles.php
+++ b/mod/profiles.php
@@ -216,7 +216,7 @@ function profiles_post(App $a) {
 		} else {
 			$ignore_year = false;
 		}
-		if (!in_array($dob, ['0000-00-00', '0001-01-01'])) {
+		if (!in_array($dob, ['0000-00-00', DBA::NULL_DATE])) {
 			if (strpos($dob, '0000-') === 0 || strpos($dob, '0001-') === 0) {
 				$ignore_year = true;
 				$dob = substr($dob, 5);
diff --git a/src/Model/Contact.php b/src/Model/Contact.php
index 1c90d0c81..f43a14e62 100644
--- a/src/Model/Contact.php
+++ b/src/Model/Contact.php
@@ -740,7 +740,7 @@ class Contact extends BaseObject
 
 			// "bd" always contains the upcoming birthday of a contact.
 			// "birthday" might contain the birthday including the year of birth.
-			if ($profile["birthday"] > '0001-01-01') {
+			if ($profile["birthday"] > DBA::NULL_DATE) {
 				$bd_timestamp = strtotime($profile["birthday"]);
 				$month = date("m", $bd_timestamp);
 				$day = date("d", $bd_timestamp);
@@ -757,7 +757,7 @@ class Contact extends BaseObject
 					$profile["bd"] = ( ++$current_year) . "-" . $month . "-" . $day;
 				}
 			} else {
-				$profile["bd"] = '0001-01-01';
+				$profile["bd"] = DBA::NULL_DATE;
 			}
 		} else {
 			$profile = $default;
@@ -794,7 +794,7 @@ class Contact extends BaseObject
 			$profile["location"] = "";
 			$profile["about"] = "";
 			$profile["gender"] = "";
-			$profile["birthday"] = '0001-01-01';
+			$profile["birthday"] = DBA::NULL_DATE;
 		}
 
 		$cache[$url][$uid] = $profile;
diff --git a/src/Model/GContact.php b/src/Model/GContact.php
index 1fed1fc3e..3acffb059 100644
--- a/src/Model/GContact.php
+++ b/src/Model/GContact.php
@@ -864,7 +864,7 @@ class GContact
 						'location' => $contact['location'], 'about' => $contact['about']];
 
 				// Don't update the birthday field if not set or invalid
-				if (empty($contact['birthday']) || ($contact['birthday'] < '0001-01-01')) {
+				if (empty($contact['birthday']) || ($contact['birthday'] <= DBA::NULL_DATE)) {
 					unset($fields['bd']);
 				}
 
diff --git a/src/Model/Profile.php b/src/Model/Profile.php
index c45bcdb84..df4c8d3c6 100644
--- a/src/Model/Profile.php
+++ b/src/Model/Profile.php
@@ -749,7 +749,7 @@ class Profile
 				$profile['gender'] = [L10n::t('Gender:'), $a->profile['gender']];
 			}
 
-			if (($a->profile['dob']) && ($a->profile['dob'] > '0001-01-01')) {
+			if (!empty($a->profile['dob']) && $a->profile['dob'] > DBA::NULL_DATE) {
 				$year_bd_format = L10n::t('j F, Y');
 				$short_bd_format = L10n::t('j F');
 
@@ -763,7 +763,7 @@ class Profile
 			}
 
 			if (!empty($a->profile['dob'])
-				&& $a->profile['dob'] > '0001-01-01'
+				&& $a->profile['dob'] > DBA::NULL_DATE
 				&& $age = Temporal::getAgeByTimezone($a->profile['dob'], $a->profile['timezone'], '')
 			) {
 				$profile['age'] = [L10n::t('Age:'), $age];
diff --git a/src/Protocol/DFRN.php b/src/Protocol/DFRN.php
index ff5081183..7c03a2183 100644
--- a/src/Protocol/DFRN.php
+++ b/src/Protocol/DFRN.php
@@ -16,7 +16,6 @@ use Friendica\Content\Text\BBCode;
 use Friendica\Content\Text\HTML;
 use Friendica\Core\Addon;
 use Friendica\Core\Config;
-use Friendica\Core\L10n;
 use Friendica\Core\Logger;
 use Friendica\Core\Protocol;
 use Friendica\Core\System;
@@ -690,7 +689,7 @@ class DFRN
 			XML::addElement($doc, $author, "poco:displayName", $profile["name"]);
 			XML::addElement($doc, $author, "poco:updated", $namdate);
 
-			if (trim($profile["dob"]) > '0001-01-01') {
+			if (trim($profile["dob"]) > DBA::NULL_DATE) {
 				XML::addElement($doc, $author, "poco:birthday", "0000-".date("m-d", strtotime($profile["dob"])));
 			}
 
@@ -1722,9 +1721,9 @@ class DFRN
 			// "poco:birthday" is the birthday in the format "yyyy-mm-dd"
 			$value = XML::getFirstNodeValue($xpath, $element . "/poco:birthday/text()", $context);
 
-			if (!in_array($value, ["", "0000-00-00", "0001-01-01"])) {
+			if (!in_array($value, ["", "0000-00-00", DBA::NULL_DATE])) {
 				$bdyear = date("Y");
-				$value = str_replace("0000", $bdyear, $value);
+				$value = str_replace(["0000", "0001"], $bdyear, $value);
 
 				if (strtotime($value) < time()) {
 					$value = str_replace($bdyear, $bdyear + 1, $value);