Basically I would like the new time variable for timepoint 1 to be coded as zero; however, each subsequent time should be the difference between date associated with timepoint 2 or timepoint 3 and timepoint 1. so the final output should as follows: ID Completed Date Timepoint Time(days) 1 2/19/2018 1 0 1 2/20/2018 2 1 1 2/21/2018 3 2 2 2/13/2018 1 0 2 2/14/2018 2 1 2 2/15/2018 3 1 3 2/15/2015 1 0 3 2/16/2015 2 1
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