date  --date="STRING"
date  --date="next Friday"
date  --date="2 days ago"
date  --date="yesterday"
date  --date="yesterday" +"%format"
# Get yesterday's date in dd-mm-yy format
date  --date="yesterday" +"%d-%m-%y"
date  --date="yesterday" +"%m-%d-%y" # US date format
date  --date="yesterday" +"%Y-%m-%d" # YYYY-mm-dd format
## store y'day date in a shell variable called yday and display it ##
yday=$(date  --date="yesterday" +"%Y-%m-%d")
echo "$yday"

• You can use fortnight for 14 day.
• week for 7 days.
• hour for 60 minutes
• minute for 60 seconds
• second for one second
• You can also use this / now / today keywords to stress the meaning.

date --date='fortnight'
date --date='5 fortnight'
date --date='fortnight ago'
date --date='5 fortnight ago'
date --date='2 hour'
date --date='2 hour ago'
date --date='20 minute'
date --date='20 minute ago'
date --date='this Friday'
## OR ##
date --date='next Friday'

Set System Date & Time?

date --set='+30 minutes'

date --set='1 day ago'