Please, read my comment to the question first!
Here is an example of what i've stated in the comment.
System.Globalization.CultureInfo[] cis = new System.Globalization.CultureInfo[]
{
new System.Globalization.CultureInfo("PL-pl"),
new System.Globalization.CultureInfo("DE-de"),
new System.Globalization.CultureInfo("EN-us")
};
DateTime dt = DateTime.Today;
foreach(System.Globalization.CultureInfo ci in cis)
Console.WriteLine(dt.ToString("d", ci));
As you can see, i'm using one date, but result is:
2019-12-10
10.12.2019
12/10/2019
More:
Standard Date and Time Format Strings | Microsoft Docs[
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Custom date and time format strings - .NET | Microsoft Docs[
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Additional note: Do not create a date like this:
DateTime dt = DateTime.Parse(DDLday.Text + "/" + DDLmonth.Text + "/" + DDLYear.Text),
Rather then it, use:
DateTime dt = new DateTime(DDLday.SelectedValue, DDLmonth.SelectedValue, DDLYear.SelectedValue),
Finally, i'd suggest to read this:
asp.net - SelectedValue vs SelectedItem.Value of DropDownList - Stack Overflow[
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