Dragan, I think you'll find this is a date convert to human readable string issue, depending on SQL Server defaults.
No doubt you also considered a parameterized query that avoids string cast and gets cached for efficiency:
ltStart={^2015-01-01}
ltend=dtot({^2016-01-01})-1
select * from mytable where mydate between ?ltStart and ?ltEnd
"... They ne'er cared for us
yet: suffer us to famish, and their store-houses
crammed with grain; make edicts for usury, to
support usurers; repeal daily any wholesome act
established against the rich, and provide more
piercing statutes daily, to chain up and restrain
the poor. If the wars eat us not up, they will; and
there's all the love they bear us."
-- Shakespeare: Coriolanus, Act 1, scene 1