This worked for me :
date -d '20121212 7 days'
date -d '12-DEC-2012 7 days'
date -d '2012-12-12 7 days'
date -d '2012-12-12 4:10:10PM 7 days'
date -d '2012-12-12 16:10:55 7 days'
then you can format output adding parameter '+%Y%m%d'
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…