diff --git a/lib/plug/crypto.ex b/lib/plug/crypto.ex
index fa8770b..65e9d14 100644
--- a/lib/plug/crypto.ex
+++ b/lib/plug/crypto.ex
@@ -168,7 +168,7 @@ defmodule Plug.Crypto do
     * `:key_digest` - option passed to `Plug.Crypto.KeyGenerator`
       when generating the encryption and signing keys. Defaults to `:sha256`
     * `:signed_at` - set the timestamp of the token in seconds.
-      Defaults to `System.system_time(:millisecond)`
+      Defaults to `System.os_time(:millisecond)`
     * `:max_age` - the default maximum age of the token. Defaults to
       `86400` seconds (1 day) and it may be overridden on `verify/4`.
 
@@ -196,7 +196,7 @@ defmodule Plug.Crypto do
     * `:key_digest` - option passed to `Plug.Crypto.KeyGenerator`
       when generating the encryption and signing keys. Defaults to `:sha256`
     * `:signed_at` - set the timestamp of the token in seconds.
-      Defaults to `System.system_time(:millisecond)`
+      Defaults to `System.os_time(:millisecond)`
     * `:max_age` - the default maximum age of the token. Defaults to
       `86400` seconds (1 day) and it may be overridden on `decrypt/4`.
 
@@ -347,5 +347,5 @@ defmodule Plug.Crypto do
   defp expired?(_signed, max_age_secs) when max_age_secs <= 0, do: true
   defp expired?(signed, max_age_secs), do: signed + trunc(max_age_secs * 1000) < now_ms()
 
-  defp now_ms, do: System.system_time(:millisecond)
+  defp now_ms, do: System.os_time(:millisecond)
 end
diff --git a/test/plug/crypto_test.exs b/test/plug/crypto_test.exs
index 9f6a738..95d2bf5 100644
--- a/test/plug/crypto_test.exs
+++ b/test/plug/crypto_test.exs
@@ -192,7 +192,7 @@ defmodule Plug.CryptoTest do
 
     test "supports signed_at in seconds" do
       seconds_in_day = 24 * 60 * 60
-      day_ago_seconds = System.system_time(:second) - seconds_in_day
+      day_ago_seconds = System.os_time(:second) - seconds_in_day
       token = encrypt(@key, "secret", 1, signed_at: day_ago_seconds)
       assert decrypt(@key, "secret", token, max_age: seconds_in_day + 1) == {:ok, 1}