[boost] How to represent underspecified Date in boost::date_time library?

Hi, recently I found boost::date_time is a great library to work with for temporal work. But now I have a problem for your help with or input. It looks like the constructor of 'date' has three parameters year, month, day. What about if I just want to construct a Date with 'year'?, because this is typical in text, like "In 1991, ...". Is this possilbe? If not, is there any way around this limitation? or even more, can I represent sth like "In the 21st century", or "the winter of 1998"? But now, for me it is important to represent dates like "1998", without any month or day mentioned. Thanks for your time.