how to find date difference in unix?
['Provide valid time string in A and B.', 'Use date -d to handle time strings.', 'Use date %s to convert time strings to seconds since 1970 (unix epoche)', 'Use bash parameter expansion to subtract seconds.', 'divide by seconds per day (86400=60*60*24) to get difference as days.', '! DST is not taken into account ! See this answer at unix.']