shell中给定日期之后向前或者后推算日期

DATE="20170414" //给定的时间 theday=`date -d "$DATE 0 days" "+%Y%m%d"` //20170414 twodaysago=`date -d "$DATE -2 days" "+%Y-%m-%d %H:%M:%S"` //2017-04-12 00:00:00 twodayslater=`date -d "$DATE 2 days" "+%Y%m%d"` //20170416