diff --git a/htdocs/comm/action/card.php b/htdocs/comm/action/card.php
index b2a149ead79..eb48d9ea7b3 100644
--- a/htdocs/comm/action/card.php
+++ b/htdocs/comm/action/card.php
@@ -270,11 +270,11 @@ if (empty($reshook) && $action == 'add') {
 	if ($fulldayevent) {
 		$tzforfullday = getDolGlobalString('MAIN_STORE_FULL_EVENT_IN_GMT');
 		// For "full day" events, we must store date in GMT (It must be viewed as same moment everywhere)
-		$datep = dol_mktime($fulldayevent ? '00' : GETPOST("aphour", 'int'), $fulldayevent ? '00' : GETPOST("apmin", 'int'), $fulldayevent ? '00' : GETPOST("apsec", 'int'), GETPOST("apmonth", 'int'), GETPOST("apday", 'int'), GETPOST("apyear", 'int'), $tzforfullday ? $tzforfullday : 'tzuser');
-		$datef = dol_mktime($fulldayevent ? '23' : GETPOST("p2hour", 'int'), $fulldayevent ? '59' : GETPOST("p2min", 'int'), $fulldayevent ? '59' : GETPOST("apsec", 'int'), GETPOST("p2month", 'int'), GETPOST("p2day", 'int'), GETPOST("p2year", 'int'), $tzforfullday ? $tzforfullday : 'tzuser');
+		$datep = dol_mktime($fulldayevent ? '00' : GETPOST("aphour", 'int'), $fulldayevent ? '00' : GETPOST("apmin", 'int'), $fulldayevent ? '00' : GETPOST("apsec", 'int'), GETPOST("apmonth", 'int'), GETPOST("apday", 'int'), GETPOST("apyear", 'int'), $tzforfullday ? $tzforfullday : 'tzuserrel');
+		$datef = dol_mktime($fulldayevent ? '23' : GETPOST("p2hour", 'int'), $fulldayevent ? '59' : GETPOST("p2min", 'int'), $fulldayevent ? '59' : GETPOST("apsec", 'int'), GETPOST("p2month", 'int'), GETPOST("p2day", 'int'), GETPOST("p2year", 'int'), $tzforfullday ? $tzforfullday : 'tzuserrel');
 	} else {
-		$datep = dol_mktime($fulldayevent ? '00' : GETPOST("aphour", 'int'), $fulldayevent ? '00' : GETPOST("apmin", 'int'), $fulldayevent ? '00' : GETPOST("apsec", 'int'), GETPOST("apmonth", 'int'), GETPOST("apday", 'int'), GETPOST("apyear", 'int'), 'tzuser');
-		$datef = dol_mktime($fulldayevent ? '23' : GETPOST("p2hour", 'int'), $fulldayevent ? '59' : GETPOST("p2min", 'int'), $fulldayevent ? '59' : GETPOST("apsec", 'int'), GETPOST("p2month", 'int'), GETPOST("p2day", 'int'), GETPOST("p2year", 'int'), 'tzuser');
+		$datep = dol_mktime($fulldayevent ? '00' : GETPOST("aphour", 'int'), $fulldayevent ? '00' : GETPOST("apmin", 'int'), $fulldayevent ? '00' : GETPOST("apsec", 'int'), GETPOST("apmonth", 'int'), GETPOST("apday", 'int'), GETPOST("apyear", 'int'), 'tzuserrel');
+		$datef = dol_mktime($fulldayevent ? '23' : GETPOST("p2hour", 'int'), $fulldayevent ? '59' : GETPOST("p2min", 'int'), $fulldayevent ? '59' : GETPOST("apsec", 'int'), GETPOST("p2month", 'int'), GETPOST("p2day", 'int'), GETPOST("p2year", 'int'), 'tzuserrel');
 	}
 
 	// Check parameters
diff --git a/htdocs/core/lib/xcal.lib.php b/htdocs/core/lib/xcal.lib.php
index 8a9824dde32..c0c762a3152 100644
--- a/htdocs/core/lib/xcal.lib.php
+++ b/htdocs/core/lib/xcal.lib.php
@@ -203,9 +203,18 @@ function build_calfile($format, $title, $desc, $events_array, $outputfile)
 				$startdatef = dol_print_date($startdate, "dayhourxcard", 'gmt');
 
 				if ($fulldayevent) {
+					// For fullday event, date was stored with old version by using the user timezone instead of storing the date at UTC+0
+					// in the timezone of server (so for a PHP timezone of -3, we should store '2023-05-31 21:00:00.000'
+					// Using option MAIN_STORE_FULL_EVENT_IN_GMT=1 change the behaviour to store in GMT for full day event. This must become
+					// the default behaviour but there is no way to change keeping old saved date compatible.
+					$tzforfullday = getDolGlobalString('MAIN_STORE_FULL_EVENT_IN_GMT');
 					// Local time should be used to prevent users in time zones earlier than GMT from being one day earlier
 					$prefix     = ";VALUE=DATE";
-					$startdatef = dol_print_date($startdate, "dayxcard", 'tzserver');
+					if ($tzforfullday) {
+						$startdatef = dol_print_date($startdate, "dayxcard", 'gmt');
+					} else {
+						$startdatef = dol_print_date($startdate, "dayxcard", 'tzserver');
+					}
 				}
 
 				fwrite($calfileh, "DTSTART".$prefix.":".$startdatef."\n");