[boost] about the Data_time; How I can init a ptime that only have the time no the date;

As I knew : construct a ptime use the ptime("2002-01-20 23:59:59.000"); but now I want to init a ptime look like ptime("23:59:59.000");when I use this funcation,a exception is show! any body can tell me ,,How Can i do?!