Estimated vs Actual Number of Rows
Up to date statistics does not necessarily imply perfect estimation. If you try to represent a range of 170 million values with 200 samples, even a small skew can throw you off by a factor of 10. 

Here is a hint: time series are almost always accessed by time. Very likely DateStamp should be clustered key. For more detailes, post the exact DDL and actual execution plan (XML, not picture).