smesh

########## func_date2days - count days from base to given date. 
#  Input: two dates like "jan/1/2017"
########## uncomment for testing
:local date $1
#:local date "sep/13/2017"
#:put $date

# Base MUST BE lower of any given date. Mikrotik counts from jan/01/1970. 
#correct only for years>1918
:local base 1970

########################################
:local mdays  {31;28;31;30;31;30;31;31;30;31;30;31}
:local months {"jan"=1;"feb"=2;"mar"=3;"apr"=4;"may"=5;"jun"=6;"jul"=7;"aug"=8;"sep"=9;"oct"=10;"nov"=11;"dec"=12}

#date 
:local dd  [:tonum [:pick $date 4 6]]
:local yy [:tonum [:pick $date 7 11]]
:local mm [:pick $date 0 3]
:set mm ($months->$mm)

#0. We have dd/mm/yy
:local res 0

#1. Move to 01/mm/yy
:set res ($res+$dd-1)

#2. Move to 01/01/yy
:while ($mm>1) do={
  :set mm ($mm-1)
  :local dm [:pick $mdays ($mm-1)]
  :if ($mm=2 && (($yy&3=0 && $yy/100*100 != $yy) || $yy/400*400=$yy) ) do={ :set dm 29 }
  :set res ($res+$dm)
}

#3. Move to 01/01/1900
:while ($yy>$base) do={
  :set yy ($yy-1)
  :set res ($res+365)
  :if (  ( ($yy&3=0) && ($yy/100*100 != $yy)) || $yy/400*400=$yy ) do={
   :set res ($res+1)
  }
}
:return $res

########## func_days2date - convert days from base to date string
#  Input: days like 18762 (may/15/2021)
########## uncomment for testing

:local days $1

#:local days 789
#:put $days

#days starts from 1
:set days ($days + 1)

# Base MUST BE lower of any given date. Mikrotik counts from jan/01/1970. 
#correct only for years>1918
:local base 1970

########################################
:local mdays  {31;28;31;30;31;30;31;31;30;31;30;31}
:local months {1="jan";2="feb";3="mar";4="apr";5="may";6="jun";7="jul";8="aug";9="sep";10="oct";11="nov";12="dec"}

#date 
:local yy $base
:local mm 0
# base year is not leap
:local daydiff 365
:local isleap false

#1. Move to 01/01/yy
:while ($days > $daydiff) do={
  :set days ($days - $daydiff)
  :set yy ($yy + 1)
  :if (  ( ($yy & 3 = 0) && ($yy/100*100 != $yy)) || $yy/400*400=$yy ) do={
    :set daydiff 366
    :set isleap true
#    :put "adjust: $yy, days: $days"
  } else={
    :set daydiff 365
    :set isleap false
  }
#  :put "yy: $yy, days: $days"
}

#2. Adjust feb 29 if leap year
#:put "mmm: $mm, days: $days, isleap: $isleap"
:if ($isleap) do={
#  :put "adjust month days"
  :set ($mdays->1) 29
}

#3. Move to 01/mm/yy
:while ($days > $mdays->$mm) do={
#  :put ($mdays->$mm)
  :set days ($days-($mdays->$mm))
  :set mm ($mm + 1)
#  :put "mm: $mm, days: $days"
}

:local month [:tostr ($mm + 1)]
:set month ($months->$month)
:if ($days < 10) do={ :set days ("0".(:tostr $days)) } else={ :set days (:tostr $days) }
:local res "$month/$days/$yy"

#:put $res
:return $res