Add Clock interface to time.h, for reading the current wall time / date.

c++/src/kj/time.c++

@@ -30,6 +30,15 @@ kj::Exception Timer::makeTimeoutException() {
   return KJ_EXCEPTION(OVERLOADED, "operation timed out");
 }
 
+Clock& nullClock() {
+  class NullClock final: public Clock {
+  public:
+    Date now() override { return UNIX_EPOCH; }
+  };
+  static NullClock NULL_CLOCK;
+  return NULL_CLOCK;
+}
+
 struct TimerImpl::Impl {
   struct TimerBefore {
     bool operator()(TimerPromiseAdapter* lhs, TimerPromiseAdapter* rhs);

c++/src/kj/time.h

@@ -61,6 +61,16 @@ using Date = Absolute<Duration, _::DateLabel>;
 constexpr Date UNIX_EPOCH = origin<Date>();
 // The `Date` representing Jan 1, 1970 00:00:00 UTC.
 
+class Clock {
+  // Interface to read the current date and time.
+public:
+  virtual Date now() = 0;
+};
+
+Clock& nullClock();
+// A clock which always returns UNIX_EPOCH as the current time. Useful when you don't care about
+// time.
+
 class Timer {
   // Interface to time and timer functionality.
   //