Those dates that are "real" dates will convert fine. If you are ONLY storing dates I would set register_dt as SmallDateTime. You would be amazed how many times we get people struggling because some "invalid" data got into varchar fields but they were supposed to be dates\ numbers.
If you have to store as varchar then at least put in all the necessary constraints to ensure they are always valid dates.
Minot thing but if register date doesn't need to be accurate to the second you could make do with smalldatetime.
for future references like this, check Books OnLine - "Cast and Convert" help, there are a chart in wich you can view all types of conversions between data types. - There are implicit conversion between datetime and varchar.