subtract days from a date in bash

one liner for mac os x:

yesterday=$(date -d "$date -1 days" +"%Y%m%d")

You are specifying the date incorrectly. Instead, say:

date --date="${dataset_date} -${date_diff} day" +%Y-%m-%d

If you need to store it in a variable, use $(...):

p_dataset_date=$(date --date="${dataset_date} -${date_diff} day" +%Y-%m-%d)