diff --git a/lib/cldr/calendar.ex b/lib/cldr/calendar.ex
index c6b3227..ecc361e 100644
--- a/lib/cldr/calendar.ex
+++ b/lib/cldr/calendar.ex
@@ -673,7 +673,7 @@ defmodule Cldr.Calendar do
 
   """
   @spec new(module(), calendar_type(), Keyword.t()) ::
-          {:ok, calendar()} | {:module_already_exists, module()}
+          {:ok, calendar()} | {:module_already_exists, module()} | {:error, String.t()}
 
   def new(calendar_module, calendar_type, config)
       when is_atom(calendar_module) and calendar_type in [:week, :month] do
@@ -860,7 +860,8 @@ defmodule Cldr.Calendar do
       %Date{calendar: Cldr.Calendar.NRF, day: 1, month: 1, year: 2019}
 
   """
-  @spec first_day_of_year(year :: year(), calendar :: calendar()) :: Date.t()
+  @spec first_day_of_year(year :: year(), calendar :: calendar()) ::
+          Date.t() | {:error, :invalid_date}
 
   def first_day_of_year(year, calendar) do
     with {:ok, date} <- Date.new(year, 1, 1, calendar) do
@@ -888,7 +889,7 @@ defmodule Cldr.Calendar do
       ~D[2019-01-01]
 
   """
-  @spec first_day_of_year(date :: date()) :: Date.t()
+  @spec first_day_of_year(date :: date()) :: Date.t() | {:error, :invalid_date}
 
   def first_day_of_year(%Date{year: year, calendar: calendar}) do
     first_day_of_year(year, calendar)
@@ -2053,7 +2054,7 @@ defmodule Cldr.Calendar do
   """
   @spec weeks_to_days(integer) :: integer
   def weeks_to_days(n) do
-    n * @days_in_a_week
+    trunc(n * @days_in_a_week)
   end
 
   @doc """
@@ -2746,7 +2747,7 @@ defmodule Cldr.Calendar do
     {:ok, backend}
   end
 
-  @spec plus(integer, integer()) :: integer()
+  @spec plus(integer, integer()) :: integer() | {:error, :invalid_date}
   @doc false
 
   def plus(value, increment) when is_integer(value) and is_integer(increment) do
@@ -2870,7 +2871,7 @@ defmodule Cldr.Calendar do
   """
 
   @spec plus(Calendar.date() | Date.Range.t(), atom(), integer(), Keyword.t()) ::
-          Calendar.date()
+          Calendar.date() | {:error, :invalid_date}
 
   def plus(date, period, increment, options \\ [])
 
@@ -3429,7 +3430,8 @@ defmodule Cldr.Calendar do
       ~D[0000-01-01]
 
   """
-  @spec date_from_iso_days(Calendar.iso_days() | iso_day_number, calendar()) :: Date.t()
+  @spec date_from_iso_days(Calendar.iso_days() | iso_day_number, calendar()) ::
+          Date.t() | {:error, :incompatible_calendars | :invalid_date}
 
   def date_from_iso_days({days, _}, calendar) do
     date_from_iso_days(days, calendar)
diff --git a/lib/cldr/calendar/backend/calendar.ex b/lib/cldr/calendar/backend/calendar.ex
index fe7b6a0..fee30bf 100644
--- a/lib/cldr/calendar/backend/calendar.ex
+++ b/lib/cldr/calendar/backend/calendar.ex
@@ -167,8 +167,14 @@ defmodule Cldr.Calendar.Backend do
         """
         @doc since: "1.25.0"
 
-        @spec localize(datetime :: Cldr.Calendar.any_date_time(), part :: Cldr.Calendar.part(), options :: Keyword.t()) ::
-                String.t() | [Cldr.Calendar.day_of_week_to_binary()] | {:error, {module(), String.t()}}
+        @spec localize(
+                datetime :: Cldr.Calendar.any_date_time(),
+                part :: Cldr.Calendar.part(),
+                options :: Keyword.t()
+              ) ::
+                String.t()
+                | [Cldr.Calendar.day_of_week_to_binary()]
+                | {:error, {module(), String.t()}}
 
         def localize(datetime, part, options \\ []) do
           options = Keyword.put(options, :backend, unquote(backend))
diff --git a/lib/cldr/calendar/backend/month_compiler.ex b/lib/cldr/calendar/backend/month_compiler.ex
index d74e357..4518972 100644
--- a/lib/cldr/calendar/backend/month_compiler.ex
+++ b/lib/cldr/calendar/backend/month_compiler.ex
@@ -403,7 +403,7 @@ defmodule Cldr.Calendar.Compiler.Month do
 
       """
       @spec days_in_month(year :: Cldr.Calendar.year(), month :: Cldr.Calendar.month()) ::
-              days :: Calendar.day() | {:error, {module(), String.t()}}
+              days :: Calendar.day() | {:error, {module(), String.t()}} | {:ambiguous, Range.t()}
 
       @impl true
       def days_in_month(year, month) do
@@ -418,7 +418,7 @@ defmodule Cldr.Calendar.Compiler.Month do
 
       """
       @spec days_in_month(month :: Cldr.Calendar.month()) ::
-              days :: Calendar.day() | {:error, {module(), String.t()}}
+              days :: Calendar.day() | {:error, {module(), String.t()}} | {:ambiguous, Range.t()}
 
       @impl true
       def days_in_month(month) do
diff --git a/lib/cldr/calendar/backend/week_compiler.ex b/lib/cldr/calendar/backend/week_compiler.ex
index 0f1e3b2..f1fe35f 100644
--- a/lib/cldr/calendar/backend/week_compiler.ex
+++ b/lib/cldr/calendar/backend/week_compiler.ex
@@ -324,7 +324,7 @@ defmodule Cldr.Calendar.Compiler.Week do
       a week in week-based calendars..
 
       """
-      @spec periods_in_year(year :: Cldr.Calendar.year()) :: Calendar.week()
+      @spec periods_in_year(year :: Cldr.Calendar.year()) :: Calendar.week() | :error
       @impl true
       def periods_in_year(year) do
         {weeks_in_year, _} = weeks_in_year(year)
@@ -377,7 +377,11 @@ defmodule Cldr.Calendar.Compiler.Week do
 
       """
       @spec days_in_month(year :: Cldr.Calendar.year(), month :: Cldr.Calendar.month()) ::
-              days :: Calendar.day() | {:error, {module(), String.t()}}
+              days ::
+              Calendar.day()
+              | {:error, {module(), String.t()}}
+              | {:ambiguous, Range.t()}
+              | {:ambiguous, [integer(), ...]}
 
       @impl true
       def days_in_month(year, month) do
@@ -392,7 +396,11 @@ defmodule Cldr.Calendar.Compiler.Week do
 
       """
       @spec days_in_month(month :: Cldr.Calendar.month()) ::
-              days :: Calendar.day() | {:error, {module(), String.t()}}
+              days ::
+              Calendar.day()
+              | {:error, {module(), String.t()}}
+              | {:ambiguous, Range.t()}
+              | {:ambiguous, [integer(), ...]}
 
       @impl true
       def days_in_month(month) do
diff --git a/lib/cldr/calendar/base/week.ex b/lib/cldr/calendar/base/week.ex
index df12648..4c92640 100644
--- a/lib/cldr/calendar/base/week.ex
+++ b/lib/cldr/calendar/base/week.ex
@@ -140,7 +140,7 @@ defmodule Cldr.Calendar.Base.Week do
   def day_of_year(year, week, day, config) when is_date(year, week, day) do
     start_of_year = first_gregorian_day_of_year(year, config)
     this_day = first_gregorian_day_of_year(year, config) + week_to_days(week) + day
-    this_day - start_of_year + 1
+    trunc(this_day - start_of_year + 1)
   end
 
   def day_of_year(year, week, day, _config) do
@@ -210,13 +210,13 @@ defmodule Cldr.Calendar.Base.Week do
   def days_in_month(year, @months_in_year, config) do
     %Config{weeks_in_month: [_, _, weeks_in_last_month]} = config
     weeks = if long_year?(year, config), do: weeks_in_last_month + 1, else: weeks_in_last_month
-    weeks * days_in_week()
+    trunc(weeks * days_in_week())
   end
 
   def days_in_month(_year, month, config) do
     %Config{weeks_in_month: weeks_in_month} = config
     month_in_quarter = Math.amod(rem(month, @months_in_quarter), @months_in_quarter)
-    Enum.at(weeks_in_month, month_in_quarter - 1) * days_in_week()
+    (Enum.at(weeks_in_month, month_in_quarter - 1) * days_in_week()) |> trunc()
   end
 
   # If the month is the last month of the year then it will be different
@@ -225,8 +225,8 @@ defmodule Cldr.Calendar.Base.Week do
 
   def days_in_month(month, %Config{weeks_in_month: [_, _, weeks_in_last_month]} = config) do
     if month == @months_in_year do
-      long_year_days = (weeks_in_last_month + 1) * days_in_week()
-      short_year_days = weeks_in_last_month * days_in_week()
+      long_year_days = trunc((weeks_in_last_month + 1) * days_in_week())
+      short_year_days = trunc(weeks_in_last_month * days_in_week())
       {:ambiguous, [short_year_days, long_year_days]}
     else
       days_in_month(@any_year, month, config)
diff --git a/lib/cldr/calendar/calendars/julian.ex b/lib/cldr/calendar/calendars/julian.ex
index 0f7046d..97b0a4d 100644
--- a/lib/cldr/calendar/calendars/julian.ex
+++ b/lib/cldr/calendar/calendars/julian.ex
@@ -227,8 +227,8 @@ defmodule Cldr.Calendar.Julian do
   @spec day_of_year(year, month, day) :: 1..366
   @impl Calendar
   def day_of_year(year, month, day) do
-    first_day = date_to_iso_days(year, 1, 1)
-    this_day = date_to_iso_days(year, month, day)
+    first_day = date_to_iso_days(year, 1, 1) |> floor()
+    this_day = date_to_iso_days(year, month, day) |> floor()
     this_day - first_day + 1
   end
 
@@ -383,7 +383,7 @@ defmodule Cldr.Calendar.Julian do
   knowning the year an error tuple is returned.
 
   """
-  @spec days_in_month(month) :: Calendar.day()
+  @spec days_in_month(month) :: Calendar.day() | {:error, :unresolved}
   @impl true
   def days_in_month(month) do
     case month do
diff --git a/lib/cldr/calendar/duration.ex b/lib/cldr/calendar/duration.ex
index 5c35a87..ed87432 100644
--- a/lib/cldr/calendar/duration.ex
+++ b/lib/cldr/calendar/duration.ex
@@ -250,7 +250,10 @@ defmodule Cldr.Calendar.Duration do
   """
 
   @spec new(from :: date_or_time_or_datetime(), to :: date_or_time_or_datetime()) ::
-          {:ok, t()} | {:error, {module(), String.t()}}
+          {:ok, t()}
+          | {:error, {module(), String.t()}}
+          | {:ambiguous, DateTime.t(), DateTime.t()}
+          | {:gap, DateTime.t(), DateTime.t()}
 
   def new(unquote(Cldr.Calendar.datetime()) = from, unquote(Cldr.Calendar.datetime()) = to) do
     with :ok <- confirm_same_time_zone(from, to),
@@ -326,7 +329,11 @@ defmodule Cldr.Calendar.Duration do
        }}
 
   """
-  @spec new(interval :: interval()) :: {:ok, t()} | {:error, {module(), String.t()}}
+  @spec new(interval :: interval()) ::
+          {:ok, t()}
+          | {:error, {module(), String.t()}}
+          | {:ambiguous, DateTime.t(), DateTime.t()}
+          | {:gap, DateTime.t(), DateTime.t()}
 
   if Code.ensure_loaded?(CalendarInterval) do
     def new(%CalendarInterval{first: first, last: last, precision: precision} = _interval)
diff --git a/lib/cldr/calendar/interval.ex b/lib/cldr/calendar/interval.ex
index 19f8631..d2677f3 100644
--- a/lib/cldr/calendar/interval.ex
+++ b/lib/cldr/calendar/interval.ex
@@ -256,13 +256,14 @@ defmodule Cldr.Calendar.Interval do
       Date.range(~D[2019-W02-1 Cldr.Calendar.ISOWeek], ~D[2019-W02-1 Cldr.Calendar.ISOWeek])
 
   """
-  @spec day(Calendar.year(), Calendar.day(), Cldr.Calendar.calendar()) :: Date.Range.t()
+  @spec day(Calendar.year(), Calendar.day(), Cldr.Calendar.calendar()) ::
+          Date.Range.t() | {:error, :invalid_date}
   @spec day(Date.t()) :: Date.Range.t()
 
   def day(%{calendar: Calendar.ISO} = date) do
     %{date | calendar: Cldr.Calendar.Gregorian}
-    |> day
-    |> coerce_iso_calendar
+    |> day()
+    |> coerce_iso_calendar()
   end
 
   def day(date) do
diff --git a/lib/cldr/calendar/kday.ex b/lib/cldr/calendar/kday.ex
index 2b461e5..acedb75 100644
--- a/lib/cldr/calendar/kday.ex
+++ b/lib/cldr/calendar/kday.ex
@@ -44,7 +44,7 @@ defmodule Cldr.Calendar.Kday do
 
   """
   @spec kday_on_or_before(Calendar.day() | Date.t(), Cldr.Calendar.day_of_week()) ::
-          Calendar.day() | Date.t()
+          Calendar.day() | Date.t() | {:error, :incompatible_calendars | :invalid_date}
 
   def kday_on_or_before(%{year: _, month: _, day: _, calendar: calendar} = date, k)
       when k in 1..7 do
@@ -90,7 +90,7 @@ defmodule Cldr.Calendar.Kday do
 
   """
   @spec kday_on_or_after(Calendar.day() | Date.t(), Cldr.Calendar.day_of_week()) ::
-          Calendar.day() | Date.t()
+          Calendar.day() | Date.t() | {:error, :incompatible_calendars | :invalid_date}
 
   def kday_on_or_after(%{year: _, month: _, day: _, calendar: calendar} = date, k)
       when k in 1..7 do
@@ -133,7 +133,7 @@ defmodule Cldr.Calendar.Kday do
 
   """
   @spec kday_nearest(Calendar.day() | Date.t(), Cldr.Calendar.day_of_week()) ::
-          Calendar.day() | Date.t()
+          Calendar.day() | Date.t() | {:error, :incompatible_calendars | :invalid_date}
 
   def kday_nearest(%{year: _, month: _, day: _, calendar: calendar} = date, k)
       when k in 1..7 do
@@ -180,7 +180,7 @@ defmodule Cldr.Calendar.Kday do
 
   """
   @spec kday_before(Calendar.day() | Date.t(), Cldr.Calendar.day_of_week()) ::
-          Calendar.day() | Date.t()
+          Calendar.day() | Date.t() | {:error, :incompatible_calendars | :invalid_date}
 
   def kday_before(%{year: _, month: _, day: _, calendar: calendar} = date, k)
       when k in 1..7 do
@@ -226,7 +226,7 @@ defmodule Cldr.Calendar.Kday do
 
   """
   @spec kday_after(Calendar.day() | Date.t(), Cldr.Calendar.day_of_week()) ::
-          Calendar.day() | Date.t()
+          Calendar.day() | Date.t() | {:error, :incompatible_calendars | :invalid_date}
 
   def kday_after(%{year: _, month: _, day: _, calendar: calendar} = date, k)
       when k in 1..7 do
@@ -275,7 +275,7 @@ defmodule Cldr.Calendar.Kday do
 
   """
   @spec nth_kday(Calendar.day() | Date.t(), integer(), Cldr.Calendar.day_of_week()) ::
-          Calendar.day() | Date.t()
+          Calendar.day() | Date.t() | {:error, :incompatible_calendars | :invalid_date}
 
   def nth_kday(%{year: _, month: _, day: _, calendar: calendar} = date, n, k)
       when k in 1..7 and is_integer(n) do
@@ -321,7 +321,7 @@ defmodule Cldr.Calendar.Kday do
 
   """
   @spec first_kday(Calendar.day() | Date.t(), Cldr.Calendar.day_of_week()) ::
-          Calendar.day() | Date.t()
+          Calendar.day() | Date.t() | {:error, :incompatible_calendars | :invalid_date}
 
   def first_kday(%{year: _, month: _, day: _, calendar: calendar} = date, k)
       when k in 1..7 do
@@ -359,7 +359,7 @@ defmodule Cldr.Calendar.Kday do
 
   """
   @spec last_kday(Calendar.day() | Date.t(), Cldr.Calendar.day_of_week()) ::
-          Calendar.day() | Date.t()
+          Calendar.day() | Date.t() | {:error, :incompatible_calendars | :invalid_date}
 
   def last_kday(%{year: _, month: _, day: _, calendar: calendar} = date, k)
       when k in 1..7 do
diff --git a/mix.exs b/mix.exs
index 1365796..2ebd2c4 100644
--- a/mix.exs
+++ b/mix.exs
@@ -25,7 +25,13 @@ defmodule Cldr.Calendar.MixProject do
         ignore_warnings: ".dialyzer_ignore_warnings",
         plt_add_apps: ~w(inets jason mix ex_cldr_currencies ex_cldr_units ex_cldr_lists
              ex_cldr_numbers calendar_interval)a,
-        flags: [:underspecs]
+        flags: [
+          :error_handling,
+          :unknown,
+          :underspecs,
+          :extra_return,
+          :missing_return
+        ]
       ],
       compilers: Mix.compilers()
     ]
diff --git a/mix/basic_behaviour.ex b/mix/basic_behaviour.ex
index 3dc70bf..e0b704f 100644
--- a/mix/basic_behaviour.ex
+++ b/mix/basic_behaviour.ex
@@ -28,7 +28,7 @@ defmodule Cldr.Calendar.Behaviour.Basic do
 
   """
   def date_to_iso_days(year, month, day) do
-    epoch() + 365 * (year - 1) + floor(year / 4) + 30 * (month - 1) + day - 1
+    floor(epoch() + 365 * (year - 1) + floor(year / 4) + 30 * (month - 1) + day - 1)
   end
 
   @doc """
diff --git a/mix/for_dialyzer.ex b/mix/for_dialyzer.ex
index 215e35b..442614e 100644
--- a/mix/for_dialyzer.ex
+++ b/mix/for_dialyzer.ex
@@ -11,5 +11,4 @@ defmodule Cldr.Calendar.ForDialyzer do
 
     MyApp.Cldr.Calendar.localize(%{month: 3}, :month, format: :wide, locale: :da)
   end
-
 end