Well, if you are sure of the format - dd/mm/yyyy with exactly that number of characters - you can do text manipulation in SQL. Treat the submitted date as a text string, then in sql pull out the parts, and put them back together again as a date....like this.
CREATE FUNCTION dbo.ConvertDate(@dateStr as varchar(20))
RETURNS datetime
AS
BEGIN
DECLARE @result as datetime
DECLARE @DayParam as varchar(2)
DECLARE @MonthParam as varchar(2)
DECLARE @YearParam as varchar(4)
Set @dayParam = substring(@dateStr,1,2)
set @monthParam = subString(@dateStr,4,2)
set @yearParam = substring(@dateStr,7,4)
Set @result = convert(datetime, @monthparam + '/' + @dayParam + '/' + @yearParam
RETURN @result
END
Then, in your update procedure, you can do like this:
CREATE Procedure dbo.Update (@dateStr)
UPDATE MyTable
SET Mydate = dbo.Convertdate(@dateStr)