Optional parts in SimpleDateFormat

I'm reading in date strings that could be with or without a time zone adjustment: yyyyMMddHHmmssz or yyyyMMddHHmmss. When a string is missing a zone, I'll treat it as GMT. I'm not seeing any way to create optional sections in a SimpleDateFormat, but maybe I'm missing something. Is there a way to do this with a SimpleDateFormat, or should I just write a new concrete DateFormat to handle this?