fix(calendar): Properly go to previous/next year

Author: kristijanhusak

lua/orgmode/objects/date.lua

@@ -537,11 +537,28 @@ end
 ---@return number
 function Date._days_of_month(date)
   local days_of = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 }
-  if date.month == 2 then
+  local month = date.month
+
+  if month == 2 then
     return Date._days_of_february(date.year)
-  else
-    return days_of[date.month]
   end
+
+  if month >= 1 and month <= 12 then
+    return days_of[month]
+  end
+
+  -- In case the month goes below or above the threshold (via adding or subtracting)
+  -- We need to adjust it to be within the range of 1-12
+  -- by either adding or subtracting
+  if month < 1 then
+    month = 12 - month
+  end
+
+  if month > 12 then
+    month = month - 12
+  end
+
+  return days_of[month]
 end
 
 function Date._days_of_february(year)

tests/plenary/object/date_spec.lua

@@ -329,6 +329,14 @@ describe('Date object', function()
     assert.are.same('2020-03-26 Thu 12:30', date:to_string())
   end)
 
+  it('properly adds and subtracts month at the edge of the year', function()
+    local date = Date.from_string('2024-01-03 Wed 14:00')
+    date = date:subtract({ month = 1 })
+    assert.are.same('2023-12-03 Sun 14:00', date:to_string())
+    date = date:add({ month = 1 })
+    assert.are.same('2024-01-03 Wed 14:00', date:to_string())
+  end)
+
   it('should compare dates', function()
     local date = Date.from_string('2021-05-12 14:00')
     local date_end = Date.from_string('2021-05-12 23:30')