Is it just a query behaviour or you see it only when wrapped into stored procedure?
Does look like parameter sniffing to me...
Also, it's a bad practice to use strings for datetime in non-ISO format. Always use YYYYMMDD (or YYYY-MM-DD)
_____________________________________________
"The only true wisdom is in knowing you know nothing"
"O skol'ko nam otkrytiy chudnyh prevnosit microsofta duh!":-D
(So many miracle inventions provided by MS to us...)
How to post your question to get the best and quick help[/url]