Add more custom date formatter tests.

- Failing: FormatsCustomMilliseconds

test/format_test.cxx

@@ -60,18 +60,30 @@ TEST(FormatTest, HandlesDateFormatter) {
   EXPECT_THAT(fmt.process({{NOW,0}}), Eq("2019-04-04 18:17:20,000"));
 }
 
-TEST(FormatTest, FormatsMicroseconds) {
+TEST(FormatTest, FormatsMilliseconds) {
   using testing::Eq;
   auto fmt = format::parse_format_string("%d");
   EXPECT_THAT(fmt.process({{NOW,123000}}), Eq("2019-04-04 18:17:20,123"));
 }
 
+TEST(FormatTest, ThrowsIfCustomFmtUnterminated) {
+  using testing::Eq;
+  EXPECT_THROW(format::parse_format_string("%d{%"),
+               format_parsing_exception);
+}
+
 TEST(FormatTest, SupportsCustomFormatWithBrace) {
   using testing::Eq;
   auto fmt = format::parse_format_string("%d{%Y}");
   EXPECT_THAT(fmt.process({{NOW,0}}), Eq("2019"));
 }
 
+TEST(FormatTest, FormatsCustomMilliseconds) {
+  using testing::Eq;
+  auto fmt = format::parse_format_string("%d{%_ms}");
+  EXPECT_THAT(fmt.process({{NOW,123000}}), Eq("123"));
+}
+
 TEST(FormatTest, SupportsISO8601Format) {
   using testing::Eq;
   auto fmt = format::parse_format_string("%d{ISO8601}");