fix(calendar): Do not modify time when switching months

lua/orgmode/objects/calendar.lua

@@ -335,7 +335,7 @@ end
 
 function Calendar:forward()
   self:_ensure_day()
-  self.date = self.date:start_of('month'):add({ month = vim.v.count1 })
+  self.date = self.date:set({ day = 1 }):add({ month = vim.v.count1 })
   self:render()
   vim.fn.cursor(2, 1)
   vim.fn.search('01')
@@ -344,7 +344,7 @@ end
 
 function Calendar:backward()
   self:_ensure_day()
-  self.date = self.date:start_of('month'):subtract({ month = vim.v.count1 }):end_of('month')
+  self.date = self.date:set({ day = 1 }):subtract({ month = vim.v.count1 }):last_day_of_month()
   self:render()
   vim.fn.cursor(8, 0)
   vim.fn.search([[\d\d]], 'b')

lua/orgmode/objects/date.lua

@@ -498,6 +498,10 @@ function Date:end_of(span)
   return self
 end
 
+function Date:last_day_of_month()
+  return self:set({ day = Date._days_of_month(os_date(self.timestamp)) })
+end
+
 ---@return number
 function Date:get_isoweekday()
   ---@type table

tests/plenary/object/date_spec.lua

@@ -303,6 +303,16 @@ describe('Date object', function()
     assert.are.same('2021-05-31 Mon 23:59', date:to_string())
   end)
 
+  it('should get last day of the month', function()
+    local date = Date.from_string('2024-02-02 09:00')
+    date = date:last_day_of_month()
+    assert.are.same('2024-02-29 Thu 09:00', date:to_string())
+    date = date:add({ month = 1 }):last_day_of_month()
+    assert.are.same('2024-03-31 Sun 09:00', date:to_string())
+    date = date:set({ month = 6, day = 2 }):last_day_of_month()
+    assert.are.same('2024-06-30 Sun 09:00', date:to_string())
+  end)
+
   it('should properly handle end of month', function()
     local date = Date.from_string('2021-05-12')
     date = date:end_of('month')