https://learn.microsoft.com/en-gb/dotnet/api/system.globalization.thaibuddhistcalendar.addmonths?view=net-9.0
ThaiBuddhistCalendar.AddMonths(DateTime, Int32) Method (System.Globalization) | Microsoft Learn
Returns a DateTime that is the specified number of months away from the specified DateTime.
datetimemethodsystemglobalizationmicrosoft