Someone

alias -l LeapYearDays { 
  set %yy $1
  set %LY $calc($date(yyyy) - %yy)

  if (%LY < 0) { return 0 }

  set %LY2 0
  set %dys 0

  while (%LY2 < %LY) {
    if (4 // $calc(%yy + %LY2)) {
      inc %dys
      inc %LY2
    }
  }
  return %dys
}