Hi @Peter_1985 ,
From your description, it seems that your application is an Asp.net Webforms application, right?
In this scenario, you could consider use the Bootstrap Datepicker or JQUery UI Datepicker. Please refer the following sample:
Using Bootstrap Datepicker:
The result as below: you can download the code from here.
Use JQuery UI DatePicker:
The result as below: you can check the code from here.
Reference:
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Best regards,
Dillion