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A new strontium ion trap of the endcap design was built recently at the National Research Council of Canada for better control of the micromotion shifts. The uncertainty caused by these shifts has been reduced by more than four orders of magnitude compared to the original NRC trap, down to a level of ≈ 10-18. In this paper we discuss the evaluation of micromotion shifts and of several other sources of importance to optical frequency standards based on single ions. The total fractional frequency uncertainty of the new strontium ion trap system is estimated to be ≈ 2 × 10-17, limited by the blackbody radiation shift. A preliminary measurement of the clock transition frequency with a fractional uncertainty of 2×10-15, limited by the accuracy of the maser reference, is also presented.