diff --git a/lib/crontab/cron_expression.ex b/lib/crontab/cron_expression.ex
index 866f378..c4b205c 100644
--- a/lib/crontab/cron_expression.ex
+++ b/lib/crontab/cron_expression.ex
@@ -8,13 +8,13 @@ defmodule Crontab.CronExpression do
   @type t :: %Crontab.CronExpression{
     extended: boolean,
     reboot: boolean,
-    second: value,
-    minute: value,
-    hour: value,
-    day: value,
-    month: value,
-    weekday: value,
-    year: value
+    second: [value],
+    minute: [value],
+    hour: [value],
+    day: [value],
+    month: [value],
+    weekday: [value],
+    year: [value]
   }
   @type interval :: :second | :minute | :hour | :day | :month | :weekday | :year
   @type min_max :: {:-, time_unit, time_unit}
diff --git a/lib/crontab/cron_expression/parser.ex b/lib/crontab/cron_expression/parser.ex
index a6aad6b..b88d92a 100644
--- a/lib/crontab/cron_expression/parser.ex
+++ b/lib/crontab/cron_expression/parser.ex
@@ -112,7 +112,7 @@ defmodule Crontab.CronExpression.Parser do
     end
   end
 
-  @spec interpret([binary], [CronExpression.interval], CronExpression.t) :: CronExpression.t | {:error, binary}
+  @spec interpret([binary], [CronExpression.interval], CronExpression.t) :: {:ok, CronExpression.t} | {:error, binary}
   defp interpret([head_format | tail_format], [head_expression | tail_expression], cron_expression) do
     conditions = interpret head_expression, head_format
     case conditions do
@@ -128,17 +128,17 @@ defmodule Crontab.CronExpression.Parser do
   defp interpret(interval, format) do
     parts = String.split(format, ",")
     tokens = Enum.map(parts, fn(part) -> tokenize interval, part end)
-    if has_failed_tokens(tokens) do
-      has_failed_tokens(tokens)
+    if get_failed_token(tokens) do
+      get_failed_token(tokens)
     else
       {:ok, Enum.map(tokens, fn({:ok, token}) -> token end)}
     end
   end
 
-  @spec has_failed_tokens([{:error, binary}] | CronExpression.value) :: boolean | binary
-  defp has_failed_tokens(tokens) do
+  @spec get_failed_token([{:error, binary}] | CronExpression.value) :: {:error, binary} | nil
+  defp get_failed_token(tokens) do
     Enum.find(tokens, fn(token) -> case token do
-      failed_token = {:error, _} -> failed_token
+      {:error, _} -> true
       _ -> false
     end end)
   end
diff --git a/lib/crontab/scheduler.ex b/lib/crontab/scheduler.ex
index edccb37..e4d7063 100644
--- a/lib/crontab/scheduler.ex
+++ b/lib/crontab/scheduler.ex
@@ -290,7 +290,6 @@ defmodule Crontab.Scheduler do
   defp reset(date = %NaiveDateTime{minute: minute}, :minutes), do: date |> reset(:seconds) |> Timex.shift(minutes: 0 - minute)
 
   @spec upper(NaiveDateTime.t, :microseconds | :seconds | :minutes) :: NaiveDateTime.t
-  defp upper(date = %NaiveDateTime{}, :microseconds), do: Map.put(date, :microsecond, {0,0})
   defp upper(date = %NaiveDateTime{second: second}, :seconds), do: date |> reset(:microseconds) |> Timex.shift(seconds: 59 - second)
   defp upper(date = %NaiveDateTime{minute: minute}, :minutes), do: date |> reset(:seconds) |> Timex.shift(minutes: 59 - minute)