That would be better to post it at Statistical Forum(let @StatDave  @lvm  @SteveDenham  see it):     Here is the code I proposed. proc mixed data=a; class id sex visit; model BMI=accel_sleep_duration sex visit child_age / solution; random int/subject= id; repeated visit/subject= id; run;     ... View more

@Jasminalves wrote: data new; set old; new_date=datepart(old_date); informat new_date ddmmyy10.; run; Attaching an INFORMAT to a variable after the values have already been stored in the variable does nothing.  If you want to impact how the value are displayed you need to attach a FORMAT to the variable.   Also if you display dates in either DMY or MDY order it will confuse half your audience.  Either use DATE or YYMMDD format to display dates and avoid the confusion. ... View more