[time] Fix Timestamp minus arithmetic when rhs is InfPast or InfFuture (#36219)

modeled after absl/time

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Closes #36219

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36219 from yijiem:fix-infinities e102d7fe21
PiperOrigin-RevId: 621287985

src/core/lib/gprpp/time.h

@@ -322,6 +322,12 @@ inline Timestamp operator-(Timestamp lhs, Duration rhs) {
 inline Timestamp operator+(Duration lhs, Timestamp rhs) { return rhs + lhs; }
 
 inline Duration operator-(Timestamp lhs, Timestamp rhs) {
+  if (rhs == Timestamp::InfPast() && lhs != Timestamp::InfPast()) {
+    return Duration::Infinity();
+  }
+  if (rhs == Timestamp::InfFuture() && lhs != Timestamp::InfFuture()) {
+    return Duration::NegativeInfinity();
+  }
   return Duration::Milliseconds(
       time_detail::MillisAdd(lhs.milliseconds_after_process_epoch(),
                              -rhs.milliseconds_after_process_epoch()));

src/core/load_balancing/weighted_round_robin/weighted_round_robin.cc

@@ -213,7 +213,7 @@ class WeightedRoundRobin final : public LoadBalancingPolicy {
     Mutex mu_;
     float weight_ ABSL_GUARDED_BY(&mu_) = 0;
     Timestamp non_empty_since_ ABSL_GUARDED_BY(&mu_) = Timestamp::InfFuture();
-    Timestamp last_update_time_ ABSL_GUARDED_BY(&mu_) = Timestamp::InfPast();
+    Timestamp last_update_time_ ABSL_GUARDED_BY(&mu_) = Timestamp::InfFuture();
   };
 
   class WrrEndpointList final : public EndpointList {

test/core/gprpp/time_test.cc

@@ -30,6 +30,17 @@ TEST(TimestampTest, Infinities) {
             Timestamp::InfFuture());
   EXPECT_EQ(Timestamp::InfPast() + Duration::Milliseconds(1),
             Timestamp::InfPast());
+  EXPECT_EQ(Timestamp::Now() - Timestamp::InfPast(), Duration::Infinity());
+  EXPECT_EQ(Timestamp::Now() - Timestamp::InfFuture(),
+            Duration::NegativeInfinity());
+  EXPECT_EQ(Timestamp::InfPast() - Timestamp::InfPast(),
+            Duration::NegativeInfinity());
+  EXPECT_EQ(Timestamp::InfFuture() - Timestamp::InfPast(),
+            Duration::Infinity());
+  EXPECT_EQ(Timestamp::InfFuture() - Timestamp::InfFuture(),
+            Duration::Infinity());
+  EXPECT_EQ(Timestamp::InfPast() - Timestamp::InfFuture(),
+            Duration::NegativeInfinity());
 }
 
 TEST(TimestampTest, ToString) {