It would reduce value of a pattern-based API as an idea, that you can specify any format with combination of special characters. But you are right: this case is

It would reduce value of a pattern-based API as an idea, that you can specify any format with combination of special characters. But you are right: this case is so special that it would make sense to protect majority of developers from misuse. If builder pattern is used for date formatters, then it could throw an exception unless certain flag is also set: var formatter = format(“ww-e-YYYY”) .withWoYCalculations() .build();