URL conversion from query in Odata services

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I have a query like this

select * from xxx where dates between '2012-10-26' and '2012-10-27'.
and did as URL like

url/?$filter=dates gt datetime'2012-10-26' and dates lt datetime'2012-10-27'
and I am getting response by using this one.

Now, I need to get by time, so I changed the URL to

&$filter=LastModifiedDate+ge+datetime'2012-09-25 23:59:59'
But I am getting an error:

{"odata.error":{"code":"","message":{"lang":"en-US","value":"Unrecognized 'Edm.DateTime' literal 'datetime'2012-09-25 12:00:00'' at '20' in 'LastModifiedDate ge datetime'2012-09-25 12:00:00''."}}}