diff --git a/include/wx/datetime.h b/include/wx/datetime.h
index e1d392a9bd..e6b271c60c 100644
--- a/include/wx/datetime.h
+++ b/include/wx/datetime.h
@@ -1145,6 +1145,9 @@ private:
     // functions
     inline bool IsInStdRange() const;
 
+    // assign the preferred first day of a week to flags, if necessary
+    void UseEffectiveWeekDayFlags(WeekFlags &flags) const;
+
     // the internal representation of the time is the amount of milliseconds
     // elapsed since the origin which is set by convention to the UNIX/C epoch
     // value: the midnight of January 1, 1970 (UTC)
diff --git a/src/common/datetime.cpp b/src/common/datetime.cpp
index 46c27b8eb7..38ddd2980d 100644
--- a/src/common/datetime.cpp
+++ b/src/common/datetime.cpp
@@ -1770,10 +1770,7 @@ wxDateTime& wxDateTime::SetToWeekDayInSameWeek(WeekDay weekday, WeekFlags flags)
         return *this;
     }
 
-    if ( flags == Default_First )
-    {
-        flags = GetCountry() == USA ? Sunday_First : Monday_First;
-    }
+    UseEffectiveWeekDayFlags(flags);
 
     // the logic below based on comparing weekday and wdayThis works if Sun (0)
     // is the first day in the week, but breaks down for Monday_First case so
@@ -1928,10 +1925,7 @@ wxDateTime::wxDateTime_t wxDateTime::GetDayOfYear(const TimeZone& tz) const
 wxDateTime::wxDateTime_t
 wxDateTime::GetWeekOfYear(wxDateTime::WeekFlags flags, const TimeZone& tz) const
 {
-    if ( flags == Default_First )
-    {
-        flags = GetCountry() == USA ? Sunday_First : Monday_First;
-    }
+    UseEffectiveWeekDayFlags(flags);
 
     Tm tm(GetTm(tz));
     wxDateTime_t nDayInYear = GetDayOfYearFromTm(tm);
@@ -2018,10 +2012,7 @@ wxDateTime::wxDateTime_t wxDateTime::GetWeekOfMonth(wxDateTime::WeekFlags flags,
     const wxDateTime dateFirst = wxDateTime(1, tm.mon, tm.year);
     const wxDateTime::WeekDay wdFirst = dateFirst.GetWeekDay();
 
-    if ( flags == Default_First )
-    {
-        flags = GetCountry() == USA ? Sunday_First : Monday_First;
-    }
+    UseEffectiveWeekDayFlags(flags);
 
     // compute offset of dateFirst from the beginning of the week
     int firstOffset;
@@ -2134,6 +2125,16 @@ wxDateTime& wxDateTime::MakeFromTimezone(const TimeZone& tz, bool noDST)
     return Subtract(wxTimeSpan::Seconds(secDiff));
 }
 
+void wxDateTime::UseEffectiveWeekDayFlags(WeekFlags &flags) const
+{
+    if ( flags == Default_First )
+    {
+        WeekDay firstDay;
+        GetFirstWeekDay(&firstDay);
+        flags = firstDay == Sun ? Sunday_First : Monday_First;
+    }
+}
+
 // ============================================================================
 // wxDateTimeHolidayAuthority and related classes
 // ============================================================================