Label StringFormat doesn't work

I am trying to format a {datetime?} property with stringFormat, but I don't know why it doesn't applied to.

this is my code

<DataGridTemplateColumn.CellTemplate>
    <DataTemplate>
        <Grid>
         开发者_高级运维   <Label Content="{Binding From, StringFormat='{}{0:dd.MM.yyyy}'}"/>
        </Grid>
    </DataTemplate>
</DataGridTemplateColumn.CellTemplate>

Am I doing something wrong?

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Put the contents of the label into a TextBlock

<Label>
    <TextBlock Text="{Binding Path=From, StringFormat='{}{0:dd.MM.yyyy}'}"/>
</Label>