Currently RadRichTextBox uses the .NET syntax for the date and time formatting of the code fields. When such strings are exported to or imported from RTF/DOCX, the date and time formatting switch should be converted according to the document format specifications (e.g. 17.16.4.1 Date and time formatting from OOXML specification). Examples of differences:

- 'tt' in .NET is converted to am/pm specifier for the current culture. In DOCX specification, this is denoted as 'am/pm'
- '/' in .NET is converted to date separator for the current culture. In DOCX specification, the symbol doesn't have special meaning, so it's always converted to '/'.

.NET date format strings: https://docs.microsoft.com/en-us/dotnet/standard/base-types/custom-date-and-time-format-strings

MS Word date format strings: 
https://support.office.com/en-us/article/insert-edit-and-view-fields-in-word-c429bbb0-8669-48a7-bd24-bab6ba6b06bb#bm9

Workaround (partial): Modify InsertDateTimeDialog to contain only formats which are compatible with MS Word.