pldate

Simple date-manipulation in Perl (to be used in shell scripts)

Might be useful on platforms (like AIX) that don't have GNU-dateutils.

It only handles dates, not times or timestamps.

It's only supported date-format is YYYYMMDD (but it has a 'printf' command)
Its internal date-representation is number of days since 1601-01-01,
supported range: 0..292192 (1601-01-01..2400-12-31).

Usage:
  pldate                     # just print the current date (localtime)
  pldate [command-list]      # execute the commands and print the result

Commands:
  today                      # set the internal variable to the current day
                             # (automatically performed at start)
  tomorrow                   # set the internal variable to the next day
  yesterday                  # set the internal variable to the previous day

  set YYYYMMDD               # go to the specifed day
  set-int N                  # the same with internal format (see above)

  add-day[s] N               # add N days (it can be negative too)
  sub-day[s] N               # subtract N days (it can be negative too)

  add-month[s] N             # add N months (it can be negative too)
  sub-month[s] N             # subtract N months (it can be negative too)
                             # note: these two might change the day-of-month

  add-year[s] N              # add N years (it can be negative too)
  sub-year[s] N              # subtract N years (it can be negative too)
                             # note: these two might change the day-of-month
                             # eg 2004-02-29 -1 year = 2003-02-28

  next-wday N                # N=0..7: go to the next Nth day of week
  next-dow  N                # synonym of the previous

  prev-wday N                # N=0..7: go to the previous Nth day of week
  prev-dow  N                # synonym of the previous

  upto-wday N                # N=0..7: go to the next Nth day of week
                             # but stay, if it is today
                             # _not_ equivalent with 'next-wday'
  upto-dow  N                # synonym of the previous

  downto-wday N              # N=0..7: go to the previous Nth day of week
                             # but stay, if it is today
  downto-dow  N              # synonym of the previous

  set-mday N                 # N=1..31: go to Nth day of the month (or the last day)
                             # N=0: same as N=1
                             # N<0: go to the abs(N)th day of the month, counting backwards from the end

  set-yday N                 # N=1..366: go to Nth day of the year (or the last day)
                             # N=0: same as N=1
                             # N<0: go to the abs(N)th day of the year, counting backwards from the end

  set-month N                # month-manipulation functions, N=1..12
  add-month[s] N             # they are analogous to wday manipulation functions
  sub-month[s] N
  next-month N
  prev-month N
  upto-month N               # Note: this might change 'mday' (day-of-month)
  downto-month N             # eg 2001-03-31 -1 month = 2001-02-28

  print                      # print the current value as %Y%m%d
  printf FMT                 # formatted print (use %Y,%y,%m,%d,%w,%j and %I for internal number)
                             # if the last command is a print/printf, the program doesn't
                             # automatically print the actual value, only a line-feed

Complete examples:
  Next Saturday:
    ./pldate today next-dow 6

  Today if it is Saturday, otherways the first Saturday after today:
    ./pldate yesterday next-dow 6
    ./pldate today upto-dow 6

  Previous Saturday:
    ./pldate today prev-dow 6

  Today if it is Saturday, otherways the previous Saturday before today:
    ./pldate tomorrow prev-dow 6
    ./pldate today downto-dow 6

  Last day of the previous month
    ./pldate today set-mday 1 sub-days 1

  First day of the next month
    ./pldate today set-mday -1 add-days 1
    ./pldate today add-month 1 set-mday 1

  Last day of the next month
    ./pldate today add-month 1 set-mday -1

  The week containing the current date (eg 19681230-19690105)
    ./pldate today \
          downto-wday 1 \
          printf %Y%m%d- \
          next-wday 7 printf %Y%m%d

  Previous year as an interval (eg 20180101-20181231)
    ./pldate today sub-year 1 set-yday 1 printf %Y%m%d- set-yday -1 print

  Current 'school-year' (September 1 - August 31):
    ./pldate downto-month 9 set-mday 1 printf '%Y%m%d-' upto-month  8 set-mday -1 print
    ./pldate downto-month 9 set-mday 1 printf '%Y%m%d-' add-months 11 set-mday -1 print

  Days since a fixed day:
     expr "$(./pldate today printf %I)" - "$(./pldate set 20010209 printf %I)"

  Set two shell-variables consistently:
    eval $(./pldate sub-days 1 printf 'YesterdayLong=%Y-%m-%d; YesterdayShort=%Y%m%d')
    echo "$YesterdayLong" "$YesterdayShort"