You can use whatever you want for the variable name -- as long as it does not conflict with a special QV variable name.
The variable "DateFormat" is a special QV name that defines the default format for dates. You can define additional variables like
SET vMyDateFormat = 'MMM-YYYY';
The vMyDateVariable will have no impact or meaning until it is used in an expression like:
Because variables cannot be used as format string in the chart Number format dialogs, it's not typical to set up exception date formats in variable strings.