Set boost::posix_time::duration to pos_infinite?

How does one set boost::posix_time::duration to positive infinite? The class contains an is_pos_infinite function, which retrieves duration::tick_type::pos_infinite for comparison, but I cannot work out 开发者_如何学Gohow to retrieve this value and use it to set the duration.

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Constructor takes some "special values": not_a_date_time, pos_infin, or neg_infin.

time_duration td(pos_infin);

Documentation for boost can be a pain to read...