BASH Time and Date

Checking NTP
ntpstat or ntpq -pn

Thanks to Cyberciti

Convert Standard Date And Time To Epoch Time
date +%s

Convert Epoch Time To Standard Date And Time
date --date='@1335309835'

TODAY
Terminal:~$ date

NUMBER OF DAYS BETWEEN TWO DATES
Terminal:~$ echo $"(( $(date --date="031122" +%s) - $(date --date="021020" +%s) ))/(60*60*24)"|bc

NUMBER OF DAYS AFTER
Terminal:~$ date -d "230 days" or date -d "24 weeks 2 days 4 hour 6 minutes"

NUMBER OF DAYS BEFORE
Terminal:~$ date -d "230 days ago" or date -d "24 weeks 2 days 4 hour 6 minutes ago"