Date, time arithmetic, math

        1) gnu date

        2) ksh ( version 93, not 88; ksh93 M 93t+ ) printf, e.g:
           printf "%(%Y-%m-%d)T\n" "yesterday"
           Many more examples ( as of 2017.07 ):
           http://blog.fpmurphy.com/2008/10/ksh93-date-manipulation.html

        3) date.pl (limited arithmetic, but "date.pl -d '- 1 day'" works)
           ( https://www.unix.com/tips-tutorials/
           239167-general-purpose-date-script.html )

        4) dconv et al, e.g. ddiff, dconv, strptime (parsing), etc.
           ( dateutils: http://www.fresse.org/dateutils/ )

        5) tm2tm (, OK in 32-bit; fails to compile in 64-bit)
           ( https://www.unix.com/shell-programming-scripting/
           146216-date-difference-between-freebsd-linux.html#post302463136 )

        6) perl custom formatting (with function POSIX::strftime, "perldoc POSIX")

        7) date-cpan.pl, cpan perl date
           ( http://cpansearch.perl.org/src/CWEST/ppt-0.14/src/date/date.jgross )

        8) For small changes, say, "yesterday" in bash, ksh
           TZ=CST+24 date +%Y%m%d
           csh, tcsh
           setenv TZ CST+24 ; date +%Y%m%d

        9) Check for legal date:
           manstat:validata ( http://oldwww.acm.org/perlman/stat/ )

        10) Check for legal date:
            validate (local perl code)

        11) Shell, ksh, date calculations:
            www.unix.com/unix-for-dummies-questions-and-answers/4870-days-elapsed-between-2-dates.html