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

From: ˧¸ç:->´úС·É (daixf_at_[hidden])
Date: 2005-01-21 02:03:57

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?!