diff --git a/lib/elixir/lib/calendar/datetime.ex b/lib/elixir/lib/calendar/datetime.ex
index b8174743bc..b75b15e6e1 100644
--- a/lib/elixir/lib/calendar/datetime.ex
+++ b/lib/elixir/lib/calendar/datetime.ex
@@ -1576,15 +1576,15 @@ defmodule DateTime do
   def diff(datetime1, datetime2, unit \\ :second)
 
   def diff(datetime1, datetime2, :day) do
-    diff(datetime1, datetime2, :second) |> div(86400)
+    diff(datetime1, datetime2, :microsecond) |> div(86_400_000_000)
   end
 
   def diff(datetime1, datetime2, :hour) do
-    diff(datetime1, datetime2, :second) |> div(3600)
+    diff(datetime1, datetime2, :microsecond) |> div(3_600_000_000)
   end
 
   def diff(datetime1, datetime2, :minute) do
-    diff(datetime1, datetime2, :second) |> div(60)
+    diff(datetime1, datetime2, :microsecond) |> div(60_000_000)
   end
 
   def diff(
diff --git a/lib/elixir/lib/calendar/naive_datetime.ex b/lib/elixir/lib/calendar/naive_datetime.ex
index 0c7af91014..2e037d4675 100644
--- a/lib/elixir/lib/calendar/naive_datetime.ex
+++ b/lib/elixir/lib/calendar/naive_datetime.ex
@@ -541,15 +541,15 @@ defmodule NaiveDateTime do
   def diff(naive_datetime1, naive_datetime2, unit \\ :second)
 
   def diff(naive_datetime1, naive_datetime2, :day) do
-    diff(naive_datetime1, naive_datetime2, :second) |> div(86400)
+    diff(naive_datetime1, naive_datetime2, :microsecond) |> div(86_400_000_000)
   end
 
   def diff(naive_datetime1, naive_datetime2, :hour) do
-    diff(naive_datetime1, naive_datetime2, :second) |> div(3600)
+    diff(naive_datetime1, naive_datetime2, :microsecond) |> div(3_600_000_000)
   end
 
   def diff(naive_datetime1, naive_datetime2, :minute) do
-    diff(naive_datetime1, naive_datetime2, :second) |> div(60)
+    diff(naive_datetime1, naive_datetime2, :microsecond) |> div(60_000_000)
   end
 
   def diff(
diff --git a/lib/elixir/lib/calendar/time.ex b/lib/elixir/lib/calendar/time.ex
index fd2ef3a84e..eaff398363 100644
--- a/lib/elixir/lib/calendar/time.ex
+++ b/lib/elixir/lib/calendar/time.ex
@@ -901,11 +901,11 @@ defmodule Time do
   def diff(time1, time2, unit \\ :second)
 
   def diff(time1, time2, :hour) do
-    diff(time1, time2, :second) |> div(3600)
+    diff(time1, time2, :microsecond) |> div(3_600_000_000)
   end
 
   def diff(time1, time2, :minute) do
-    diff(time1, time2, :second) |> div(60)
+    diff(time1, time2, :microsecond) |> div(60_000_000)
   end
 
   def diff(
diff --git a/lib/elixir/test/elixir/calendar/datetime_test.exs b/lib/elixir/test/elixir/calendar/datetime_test.exs
index 4aa19afdc3..48605482f5 100644
--- a/lib/elixir/test/elixir/calendar/datetime_test.exs
+++ b/lib/elixir/test/elixir/calendar/datetime_test.exs
@@ -817,6 +817,7 @@ defmodule DateTimeTest do
         |> DateTime.add(-1, :microsecond)
 
       assert DateTime.diff(in_almost_7_days, datetime, :day) == 6
+      assert DateTime.diff(datetime, in_almost_7_days, :day) == -6
     end
 
     test "in microseconds" do
diff --git a/lib/elixir/test/elixir/calendar/naive_datetime_test.exs b/lib/elixir/test/elixir/calendar/naive_datetime_test.exs
index 7c2f02f21a..3644538cf3 100644
--- a/lib/elixir/test/elixir/calendar/naive_datetime_test.exs
+++ b/lib/elixir/test/elixir/calendar/naive_datetime_test.exs
@@ -238,6 +238,12 @@ defmodule NaiveDateTimeTest do
                :day
              ) == 0
 
+      assert NaiveDateTime.diff(
+               ~N[2000-01-01 00:00:00.700000],
+               ~N[2000-01-02 00:00:00.200000],
+               :day
+             ) == 0
+
       assert NaiveDateTime.diff(
                ~N[2000-01-01 00:00:00.001200],
                ~N[2000-01-01 00:00:00.000700],
diff --git a/lib/elixir/test/elixir/calendar/time_test.exs b/lib/elixir/test/elixir/calendar/time_test.exs
index 2818e75511..deb8c59f41 100644
--- a/lib/elixir/test/elixir/calendar/time_test.exs
+++ b/lib/elixir/test/elixir/calendar/time_test.exs
@@ -113,6 +113,12 @@ defmodule TimeTest do
     assert Time.diff(time1_holocene, time2, :second) == -17883
     assert Time.diff(time1_holocene, time2, :millisecond) == -17_882_889
     assert Time.diff(time1_holocene, time2, :microsecond) == -17_882_889_000
+
+    # Two times of day are always less than 24 hours apart, so neither
+    # direction can amount to a full day.
+    assert Time.diff(~T[23:59:59.999999], ~T[00:00:00.000001], :hour) == 23
+    assert Time.diff(~T[00:00:00.000001], ~T[23:59:59.999999], :hour) == -23
+    assert Time.diff(~T[00:00:00.000001], ~T[23:59:59.999999], :minute) == -1439
   end
 
   test "truncate/2" do