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.
Dess | Tech Support Engineer, Sr.
Progress is here for your business, like always. Read more
about the measures we are taking to ensure business continuity and help fight the COVID-19 pandemic.
Our thoughts here at Progress are with those affected by the outbreak.