Allow empty sigils as a syntactic sugar for UTC “now”

lib/elixir/lib/kernel.ex

@@ -5635,9 +5635,17 @@ defmodule Kernel do
       iex> ~D[2015-01-13]
       ~D[2015-01-13]
 
+  `~D` might be also called with empty content, returning the current date in UTC.
+  It’s a syntactic sugar for `Date.utc_today/0`.
   """
   defmacro sigil_D(date_string, modifiers)
 
+  defmacro sigil_D({:<<>>, _, [""]}, []) do
+    quote do
+      Date.utc_today()
+    end
+  end
+
   defmacro sigil_D({:<<>>, _, [string]}, []) do
     {{:ok, {year, month, day}}, calendar} = parse_with_calendar!(string, :parse_date, "Date")
     to_calendar_struct(Date, calendar: calendar, year: year, month: month, day: day)
@@ -5675,9 +5683,30 @@ defmodule Kernel do
       iex> ~T[13:00:07.001]
       ~T[13:00:07.001]
 
+  `~T` might be called with empty content to get to the current UTC time,
+  with the precision specified by a modifier.
+
+  ## Examples
+      iex> now_without_milliseconds = ~T[]s
+      ...> now_without_milliseconds.microsecond
+      {0, 0}
+
   """
   defmacro sigil_T(time_string, modifiers)
 
+  defmacro sigil_T({:<<>>, _, [""]}, modifiers) do
+    truncate_to =
+      case modifiers do
+        [?s] -> :second
+        [?m] -> :millisecond
+        [] -> :microsecond
+      end
+
+    quote bind_quoted: [truncate_to: truncate_to] do
+      Time.utc_now() |> Time.truncate(truncate_to)
+    end
+  end
+
   defmacro sigil_T({:<<>>, _, [string]}, []) do
     {{:ok, {hour, minute, second, microsecond}}, calendar} =
       parse_with_calendar!(string, :parse_time, "Time")
@@ -5780,10 +5809,30 @@ defmodule Kernel do
       iex> ~U[2015-01-13T13:00:07.001+00:00]
       ~U[2015-01-13 13:00:07.001Z]
 
+  `~U` might be called with empty content to get to the current UTC datetime,
+  with the precision specified by a modifier.
+
+  ## Examples
+      iex> now_without_milliseconds = ~U[]s
+      ...> now_without_milliseconds.microsecond
+      {0, 0}
   """
   @doc since: "1.9.0"
   defmacro sigil_U(datetime_string, modifiers)
 
+  defmacro sigil_U({:<<>>, _, [""]}, modifiers) do
+    truncate_to =
+      case modifiers do
+        [?s] -> :second
+        [?m] -> :millisecond
+        [] -> :microsecond
+      end
+
+    quote bind_quoted: [truncate_to: truncate_to] do
+      DateTime.utc_now() |> DateTime.truncate(truncate_to)
+    end
+  end
+
   defmacro sigil_U({:<<>>, _, [string]}, []) do
     {{:ok, {year, month, day, hour, minute, second, microsecond}, offset}, calendar} =
       parse_with_calendar!(string, :parse_utc_datetime, "UTC DateTime")

lib/elixir/test/elixir/calendar/date_test.exs

@@ -13,6 +13,10 @@ defmodule DateTest do
     assert ~D[20001-01-01 Calendar.Holocene] ==
              %Date{calendar: Calendar.Holocene, year: 20001, month: 1, day: 1}
 
+    {now1, now2} = {Date.utc_today(), ~D[]}
+    # to be safe running at midnight
+    assert Date.diff(now1, now2) <= 1
+
     assert_raise ArgumentError,
                  ~s/cannot parse "2000-50-50" as Date for Calendar.ISO, reason: :invalid_date/,
                  fn -> Code.eval_string("~D[2000-50-50]") end

lib/elixir/test/elixir/calendar/time_test.exs

@@ -13,6 +13,9 @@ defmodule TimeTest do
     assert ~T[12:34:56 Calendar.Holocene] ==
              %Time{calendar: Calendar.Holocene, hour: 12, minute: 34, second: 56}
 
+    {now1, now2} = {Time.utc_now(), ~T[]}
+    assert Time.diff(now1, now2) <= 2
+
     assert_raise ArgumentError,
                  ~s/cannot parse "12:34:65" as Time for Calendar.ISO, reason: :invalid_time/,
                  fn -> Code.eval_string("~T[12:34:65]") end

lib/elixir/test/elixir/kernel_test.exs

@@ -1399,6 +1399,9 @@ defmodule KernelTest do
              zone_abbr: "UTC"
            }
 
+    {now1, now2} = {DateTime.utc_now(), ~U[]}
+    assert DateTime.diff(now1, now2) <= 2
+
     assert_raise ArgumentError, ~r"reason: :invalid_format", fn ->
       Code.eval_string(~s{~U[2015-01-13 13:00]})
     end