متاسفانه کسی پاسخی نداشت، اما پاسخ رو اینجا میگذارم:
<Grid>
<CalendarName="MonthlyCalendar"
SelectionMode="SingleRange"
Margin="15,39,312,109"
SelectedDate="{Binding ElementName=myDatePicker, Path=SelectedDate, StringFormat='dd/MM/yyyy'}">
</Calendar>
<DatePicker
SelectedDate="{Binding Path = SellStartDate, StringFormat = {}{0:dd-MM-yyyy}}"
HorizontalAlignment="Left"Margin="273,39,0,0"Name="myDatePicker"
SelectedDateChanged="myDatePicker_SelectedDateChanged"
VerticalAlignment="Top"Height="39"Width="120">
</DatePicker>
<Grid>