1. hwclock: query or set the hardware clock (RTC)

         -s, --hctosys        set the system time from the hardware clock

         -w, --systohc        set the hardware clock from the current system time


2. data: print or set the system date and time

        -s, --set=STRING:set time described by STRING

        -d --date=STRING: display time described by STRING, not 'now'

            %d     day of month (e.g., 01)

            %D     date; same as %m/%d/%y

            %F     full date; same as %Y-%m-%d

            %H     hour (00..23)

            %m     month (01..12)

            %M     minute (00..59)

            %S     second (00..60)

            %s     seconds since 1970-01-01 00:00:00 UTC

            %Y     year

example:

    date -d "20 minutes ago" "+%m/%d/%Y %H:%M:%S"


3. cal: calendar