The following query does not update the datetime field:
update table
SET EndDate = \'2009-05-25\'
WHERE Id = 1
I also tried it with no dashes,
If you aren't interested in specifying a time, you can also use the format 'DD/MM/YYYY', however I would stick to a Conversion method, and its relevant ISO format, as you really should avoid using default values.
Here's an example:
SET startDate = CONVERT(datetime,'2015-03-11T23:59:59.000',126)
WHERE custID = 'F24'