Persian RadDateTimePicker

Hello,       
  
Since the product of this thread is specified as RadScheduler, I suppose that your question is related to RadScheduler and the RadDateTimePicker that shows in the dialogues. We already had a feature request for "fa-IR" culture and PersianCalendar support. It has already been implemented. You can refer to the following feedback item: https://feedback.telerik.com/winforms/1371834-add-radscheduler-support-for-fa-ir-culture-and-persiancalendar
It was introduced in R1 2017 version. Feel free to use at least this version or a newer one in order to benefit from the new functionality.

As it is noted in the following help article about RadDateTimePicker, there is a known issue in the .NET Framework considering the "fa-IR" culture Fixing Persian Locale for Persian Calendar and Month Names: https://docs.telerik.com/devtools/winforms/controls/editors/datetimepicker/internationalization/cultureinfo-and-regioninfo-basics
  
I hope this information helps.

Regards,
Dess | Tech Support Engineer, Sr.
Progress Telerik

I use R3 2019 Version.

I used rad datetime picker and change the culture to "fa-IR" to use in Persian format it works fine. The problem is on some of the clients, it shows the control in "en-US" format. What is the issue? I change the culture in form load event.

here is the code I use:

Hello, Javad, 

Following the provided code snippet, I have prepared a sample project to test the behavior locally. It seems that the fa-IR culture is applied to the RadDateTimePicker control on my end (OS Win 10):

According to the provided brief information, it wouldn't be easy to determine what causes the undesired behavior on your clients' machines. But if it works OK on your end, it seems to be dependent to the operating system. If your clients are using Windows version lower than Windows 10, there is a known issue in the .NET Framework considering the "fa-IR" culture. After some research in general programming forums, I have found the following article which may be suitable for your case: https://www.codeproject.com/Articles/256104/Using-Custom-Culture-Approach-to-Fix-Persian-Cutur 

I hope this information helps.

Regards,
Dess | Tech Support Engineer, Sr.
Progress Telerik