Sounds like you want to get to the day of the month.
day(datepart(datetime_var))
And generate a new variable with 3 values.
day=day(datepart(datetime_var));
if 0<day<=10 then month_part=1;
else if day <=20 then month_part=2;
else if day <=31 then month_part=3;
else month_part=.;
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