Fixed absolute values, correct subsecond width and zero padding

include/fmt/chrono.h

@@ -405,18 +405,34 @@ inline size_t strftime(wchar_t* str, size_t count, const wchar_t* format,
 
 FMT_END_DETAIL_NAMESPACE
 
-template <typename Char, typename Duration>
-struct formatter<std::chrono::time_point<std::chrono::system_clock, Duration>,
+template <class Rep, class Period, class = std::enable_if<
+   std::chrono::duration<Rep, Period>::min() < std::chrono::duration<Rep, Period>::zero()>>
+constexpr std::chrono::duration<Rep, Period> abs(std::chrono::duration<Rep, Period> d)
+{
+    return d >= d.zero() ? d : -d;
+}
+
+template <typename Char, typename Rep, typename Period>
+struct formatter<std::chrono::time_point<std::chrono::system_clock, std::chrono::duration<Rep, Period>>,
                  Char> : formatter<std::tm, Char> {
   template <typename FormatContext>
   auto format(std::chrono::time_point<std::chrono::system_clock> val,
               FormatContext& ctx) -> decltype(ctx.out()) {
     std::tm time = localtime(val);
     auto epoch = val.time_since_epoch();
-    auto seconds = std::chrono::duration_cast<std::chrono::milliseconds>(epoch).count() / 1000;
-    auto today = std::chrono::high_resolution_clock::duration(std::chrono::milliseconds(seconds * 1000));
-    auto subseconds = std::chrono::duration_cast<std::chrono::nanoseconds>(epoch - today);
-    return formatter<std::tm, Char>::format(time, ctx, subseconds.count());
+    auto seconds = std::chrono::duration_cast<std::chrono::seconds>(epoch);
+    auto subseconds = abs(std::chrono::duration_cast<std::chrono::duration<Rep, Period>>(epoch - seconds));
+
+    if (subseconds.count() > 0) {
+      auto width = std::to_string(Period::den).size() - 1;
+      std::basic_stringstream<Char> os;
+      os.fill('0');
+      os.width(width);
+      os << subseconds.count();
+      auto formatted_ss = os.str();
+      return formatter<std::tm, Char>::format(time, ctx, formatted_ss);
+    }
+    return formatter<std::tm, Char>::format(time, ctx);
   }
 };
 
@@ -432,7 +448,7 @@ template <typename Char> struct formatter<std::tm, Char> {
   }
 
   template <typename FormatContext>
-  auto format(const std::tm& tm, FormatContext& ctx, int subseconds = 0) const
+  auto format(const std::tm& tm, FormatContext& ctx, const std::basic_string<Char> subseconds = {}) const
       -> decltype(ctx.out()) {
     basic_memory_buffer<Char> tm_format;
     tm_format.append(specs.begin(), specs.end());
@@ -441,8 +457,9 @@ template <typename Char> struct formatter<std::tm, Char> {
     // https://github.com/fmtlib/fmt/issues/2238
     auto hasS = std::find(tm_format.begin(), tm_format.end(), 'S') != tm_format.end();
     auto hasT = std::find(tm_format.begin(), tm_format.end(), 'T') != tm_format.end();
-    auto writeSubseconds = (hasS || hasT) && (subseconds != 0);
+    auto writeSubseconds = (hasS || hasT) && (subseconds.size() > 0);
     if (writeSubseconds)
+      // should be std::use_facet<std::numpunct<Char>>(locale).decimal_point();
       tm_format.push_back('.');
     else
       tm_format.push_back(' ');
@@ -460,12 +477,14 @@ template <typename Char> struct formatter<std::tm, Char> {
       buf.reserve(buf.capacity() + (size > MIN_GROWTH ? size : MIN_GROWTH));
     }
 
+    auto buf_end = buf.end() - 1;
     if (writeSubseconds) {
-      buf.append(std::to_string(subseconds));
+      buf.append(subseconds);
+      buf_end = buf.end();
     }
 
     // Remove the extra space.
-    return std::copy(buf.begin(), buf.end() - 1, ctx.out());
+    return std::copy(buf.begin(), buf_end, ctx.out());
   }
 
   basic_string_view<Char> specs;

test/chrono-test.cc

@@ -100,6 +100,10 @@ TEST(chrono_test, time_point) {
       std::chrono::time_point<std::chrono::system_clock, std::chrono::seconds>;
   auto t2 = time_point(std::chrono::seconds(42));
   EXPECT_EQ(strftime(t2), fmt::format("{:%Y-%m-%d %H:%M:%S}", t2));
+  using time_point_2 = std::chrono::time_point<std::chrono::system_clock,
+                                               std::chrono::milliseconds>;
+  auto t3 = time_point_2(std::chrono::milliseconds(1023));
+  EXPECT_EQ("01.023", fmt::format("{:%S}", t3));
 }
 
 #ifndef FMT_STATIC_THOUSANDS_SEPARATOR