[Boost-users] [Date Time] time_period has not default constructor

I am using the library Date Time, in particular the object time_period. I was wondering why (if there is a good reason) this class has not a default constructor. the default constructor result usefull when using for example a map with the operator [] or when using serialization from Boost. ptime class has default constructor. If a time_period is formed by two ptime objects, why time_period has not its default constructor as well. Thanks