The variables for day names work similarly. There are two more date variables that are set by the VCL, but they are not connected with the Windows locale information. If you d like more information on this variable, look in the online help under the topic Y2K Issues. You can use it to add or subtract months from a date. It takes a TDateTime value argument and an int argument that represents the number of months to increment or decrement.
Positive numbers add months and negative numbers subtract months.
This is because June has only 30 days. When you think about it, though, this is the correct behavior. This function is not as useful as it may sound, because it is very particular about the format of the string. The separator character in the date string must match the separator contained in the DateSeparator variable or an exception is thrown. There are also functions in Sysutils that work on both the date and time portions of a TDateTime object.