I've had the same problem and determined that this issue arises because SQL Server does not perform comparisons on characters converted to integers in an identical manner. In my test, I've found that some comparisons of converted characters, such as the exclamation point, will return type conversion errors, while other comparisons of converted characters, such as the space, will be determined to be out of range.
This sample code tests the different possible scenarios and presents a solution using nested REPLACE statements. The REPLACE determines if there are any characters in the string that are not numerals or the slash, and, if any exist, the length of the string will be greater than zero, thereby indicating that there are 'bad' characters and the date is invalid.
DECLARE @str varchar(10)
SET @str = '12/10/2012'
IF convert(int, substring(@str,4,2)) <= 31 AND convert(int, substring(@str,4,2)) >= 1
PRINT @str+': Passed Test'
ELSE PRINT @str+': Failed Test'
GO
DECLARE @str varchar(10)
SET @str = '12/10/2012'
PRINT 'Number of characters in ' + @str + ' that are not numerals or a slash (0 means the date is valid; all values greater than 0 indicate a problem): ' + convert(varchar(5),len(replace(replace(replace(replace(replace(replace(replace(replace(replace(replace(replace(replace(@str,'0',''),'1',''),'2',''),'3',''),'4',''),'5',''),'6',''),'7',''), '8',''),'9',''),'/',''),' ','+'))) --replace space with a + to avoid empty string
PRINT ''
GO
DECLARE @str varchar(10)
SET @str = '12/!0/2012'
IF convert(int, substring(@str,4,2)) <= 31 AND convert(int, substring(@str,4,2)) >= 1
PRINT @str+': Passed Test'
ELSE PRINT @str+': Failed Test'
GO
DECLARE @str varchar(10)
SET @str = '12/!0/2012'
PRINT 'Number of characters in ' + @str + ' that are not numerals or a slash (0 means the date is valid; all values greater than 0 indicate a problem): ' + convert(varchar(5),len(replace(replace(replace(replace(replace(replace(replace(replace(replace(replace(replace(replace(@str,'0',''),'1',''),'2',''),'3',''),'4',''),'5',''),'6',''),'7',''), '8',''),'9',''),'/',''),' ','+'))) --replace space with a + to avoid empty string
PRINT ''
GO
DECLARE @str varchar(10)
SET @str = '12/ /2012'
IF convert(int, substring(@str,4,2)) <= 31 AND convert(int, substring(@str,4,2)) >= 1
PRINT @str+': Passed Test'
ELSE PRINT @str+': Failed Test'
GO
DECLARE @str varchar(10)
SET @str = '12/ /2012'
PRINT 'Number of characters in ' + @str + ' that are not numerals or a slash (0 means the date is valid; all values greater than 0 indicate a problem): ' + convert(varchar(5),len(replace(replace(replace(replace(replace(replace(replace(replace(replace(replace(replace(replace(@str,'0',''),'1',''),'2',''),'3',''),'4',''),'5',''),'6',''),'7',''), '8',''),'9',''),'/',''),' ','+'))) --replace space with a + to avoid empty string
Output:
--Output
--12/10/2012: Passed Test
--Number of characters in 12/10/2012 that are not numerals or a slash (0 means the date is valid; all values greater than 0 indicate a problem): 0
--Msg 245, Level 16, State 1, Line 4
--Conversion failed when converting the varchar value '!0' to data type int.
--Number of characters in 12/!0/2012 that are not numerals or a slash (0 means the date is valid; all values greater than 0 indicate a problem): 1
--12/ /2012: Failed Test
--Number of characters in 12/ /2012 that are not numerals or a slash (0 means the date is valid; all values greater than 0 indicate a problem): 2