The "R" or "r" standard format specifier represents a custom date and time format string that is defined by the DateTimeFormatInfo. RFCPattern property. The pattern reflects a defined standard, and the property is read-only. Therefore, it is always the same, regardless of the culture used or the format provider supplied. When this standard format specifier is used, the formatting or parsing operation always uses the invariant culture. InvariantInfo property that represents the invariant culture.