date/time or date and time

I am setting up an SQL database and I will need to get differences in dates. For example I have a start date, start time, completion date and completion time and I want to get the difference between the start and completion.
Would it be better to have one field with both date and time in it, or better to have a date field and a time field?
Even though I have already started setting up the tables with seperate fields for date and time I am now leaning toward one field with date/time in it. (Only because that is the way I had to do it when setting up an Excel spreadsheet for a similar task)

When that is decided could someone please point me to a good resource for explaining to me the iConvertible method. I tried a simple asp.net page to insert a record into the database and got an error method telling me I had to use the iConvertible method. (I am programming in C#). I use a textbox on a webform to input the date and time information and the SQL database fields are set up as date/time. I looked at the visual studio documentation but that doesn't help me much. It doesn't show me the syntax required and how to "use" the method.
Thanksone field with datetime should be good...easier for maintenance too..and it will solve your puspose too...

** no idea abt iConvertible...sorry

hth|||You can use DataType as DateTime and to get the difference between the Dates you can use theDateTime.Subtract()