In its current format it does not recognize the difference. If you chance the line

IF (CHARINDEX ('MON',@DateValue)>0)

SET @DateValue = REPLACE(@DateValue, 'MON',

LEFT(UPPER(DATENAME(MM, @Datetime)),3)) with

IF (CHARINDEX ('MON',@DateValue COLLATE LATIN1_GENERAL_CS_AS)>0)

SET @DateValue = REPLACE(@DateValue, 'MON',

LEFT(UPPER(DATENAME(MM, @Datetime)),3))

That will force the function to distinguish between the upper case and the lower case. Tested and works on my database.

Thank you for your comment and question.