ora-01830

Oracle date time

I have four fields: Start_Date = 16-Jun-10 Start_Time = 14:37 End_Date = 16-Jun-10 End_Time = 14:38 I need to calculate the difference from start to end. I tried using the to_date(Start_date || Start_Time) but keep getting this error message: ORA-01830: date format picture ends before converting entire input string 01830.00000 ...