Add max()/min() factories for all civil_time types.

include/civil_time_detail.h

@@ -275,6 +275,14 @@ class civil_time {
                                   preserves_data<U, T>* = nullptr) noexcept
       : civil_time(ct.f_) {}
 
+  // Factories for the maximum/minimum representable civil_time.
+  static civil_time max() {
+    return civil_time(std::numeric_limits<int>::max(), 12, 31, 23, 59, 59);
+  }
+  static civil_time min() {
+    return civil_time(std::numeric_limits<int>::min(), 1, 1, 0, 0, 0);
+  }
+
   // Field accessors.
   CONSTEXPR_M int year() const noexcept { return f_.y; }
   CONSTEXPR_M int month() const noexcept { return f_.m; }

src/civil_time_test.cc

@@ -363,6 +363,25 @@ TEST(CivilTime, FieldsConstructionLimits) {
       Format(civil_second(1970, kIntMin, kIntMin, kIntMin, kIntMin, kIntMin)));
 }
 
+TEST(CivilTime, RangeLimits) {
+  const int kIntMax = std::numeric_limits<int>::max();
+  const int kIntMin = std::numeric_limits<int>::min();
+
+  EXPECT_EQ(civil_year(kIntMax), civil_year::max());
+  EXPECT_EQ(civil_month(kIntMax, 12), civil_month::max());
+  EXPECT_EQ(civil_day(kIntMax, 12, 31), civil_day::max());
+  EXPECT_EQ(civil_hour(kIntMax, 12, 31, 23), civil_hour::max());
+  EXPECT_EQ(civil_minute(kIntMax, 12, 31, 23, 59), civil_minute::max());
+  EXPECT_EQ(civil_second(kIntMax, 12, 31, 23, 59, 59), civil_second::max());
+
+  EXPECT_EQ(civil_year(kIntMin), civil_year::min());
+  EXPECT_EQ(civil_month(kIntMin, 1), civil_month::min());
+  EXPECT_EQ(civil_day(kIntMin, 1, 1), civil_day::min());
+  EXPECT_EQ(civil_hour(kIntMin, 1, 1, 0), civil_hour::min());
+  EXPECT_EQ(civil_minute(kIntMin, 1, 1, 0, 0), civil_minute::min());
+  EXPECT_EQ(civil_second(kIntMin, 1, 1, 0, 0, 0), civil_second::min());
+}
+
 TEST(CivilTime, ImplicitCrossAlignment) {
   civil_year year(2015);
   civil_month month = year;