Hi,
The 'isValid' property of the DropDown Calendar Control is for Check if the text is valid for this position based on the mask. There is no equivalent Client side property for isValid. But you can customize the format of the TextBox using 'CustomFormat' attribute. Please refer the following code which illustrates this:
Format="CustomString"> Also please refer to the sample in the following link:
http://websamples.syncfusion.com/samples/Tools.Web/5.1.0.51/F63795/main.htmPlease let me know if you have any further queries.
Regards,
S.Rajarajeswari