Operating system error 123?

Question

si to all,
i want to backup particular database with format of "date_time_filename.bak"

Ex:2011-10-24_12:30Pm_temp.bak
when i am trying like above,i am getting 'could not open device,operating system error 123(syntax is wrong). error

when i am getting 'date_filename.bak' format,it will comes correctly but when i am add time also ,i am getting above error


plz help me,urgent
Thanks inadvance