Checking Contents Of Column And Replacing Contents If First Character Is A Letter

Hi All,
I have come up against a wall which i cannot get over.
I have an sql db where the date column is set as a varchar (i know, should have used datetime but this was done before my time and i've got to work with what is there). The majority of values are in the format dd/mm/yyyy. However, some values contain the word 'various'.
I'm attempting to compare the date chosen on a c# .net page with the values in the db and also return all the 'various' values as well.
I have accomplished casting the varchar to a datetime and then comparing to the selected date on the .net page. However, it errors when it comes across the 'various' entrant.
Is there anyway to carry out a select statement comparing the start_date values in the db to the selected date on the .net page and also pull out all 'various' entrants at the same time without it erroring? i thought about replacing the 'various' to a date like '01/01/2010' so it doesn't stumble over the none recognised format, but am unsure of how to do it.

This is how far i have got: casting the varchar column to datetime and comparing.  
SELECT * FROM table1 WHERE Cast(SUBSTRING(Start_Date,4,2) + '/' + SUBSTRING(Start_Date,1,2) + '/' +SUBSTRING(Start_Date,7,4) as datetime)  '" + date + "'"
Many thanks in advance!