diff --git a/include/fmt/chrono.h b/include/fmt/chrono.h
index f8e0d2a7ec36..38615ffc044c 100644
--- a/include/fmt/chrono.h
+++ b/include/fmt/chrono.h
@@ -388,6 +388,7 @@ template <typename T, FMT_ENABLE_IF(std::is_floating_point<T>::value)>
 inline bool isnan(T value) {
   return std::isnan(value);
 }
+
 template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
 inline bool isfinite(T) {
   return true;
@@ -396,6 +397,8 @@ template <typename T, FMT_ENABLE_IF(std::is_floating_point<T>::value)>
 inline bool isfinite(T value) {
   return std::isfinite(value);
 }
+
+//paul - is this fcn unused?
 template <typename T> inline int to_int(T value) {
   FMT_ASSERT(!isnan(value), "nan to int conversion is UB");
   if (std::numeric_limits<T>::is_signed) {
@@ -409,6 +412,20 @@ template <typename T> inline int to_int(T value) {
   return static_cast<int>(value);
 }
 
+// Convers value to int and checks that it's in the range [0, upper).
+template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
+inline int to_nonnegative_int(T value, int upper) {
+  FMT_ASSERT(value >= 0 && value <= upper, "invalid value");
+  return static_cast<int>(value);
+}
+template <typename T, FMT_ENABLE_IF(!std::is_integral<T>::value)>
+inline int to_nonnegative_int(T value, int upper) {
+  FMT_ASSERT(
+      std::isnan(value) || (value >= 0 && value <= static_cast<T>(upper)),
+      "invalid value");
+  return static_cast<int>(value);
+}
+
 template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
 inline T mod(T x, int y) {
   return x % y;
@@ -560,9 +577,9 @@ struct chrono_formatter {
 
   std::tm time() const {
     auto time = std::tm();
-    time.tm_hour = to_int(hour());
-    time.tm_min = to_int(minute());
-    time.tm_sec = to_int(second());
+    time.tm_hour = to_nonnegative_int(hour(), 24);
+    time.tm_min = to_nonnegative_int(minute(), 60);
+    time.tm_sec = to_nonnegative_int(second(), 60);
     return time;
   }
 
@@ -577,7 +594,8 @@ struct chrono_formatter {
     write_sign();
     if (isnan(value)) return write_nan();
     typedef typename int_traits<int>::main_type main_type;
-    main_type n = to_unsigned(to_int(value));
+    main_type n = to_unsigned(
+        to_nonnegative_int(value, (std::numeric_limits<int>::max)()));
     int num_digits = internal::count_digits(n);
     if (width > num_digits) out = std::fill_n(out, width - num_digits, '0');
     out = format_decimal<char_type>(out, n, num_digits);
@@ -624,7 +642,7 @@ struct chrono_formatter {
 
     if (ns == numeric_system::standard) return write(hour(), 2);
     auto time = tm();
-    time.tm_hour = to_int(hour());
+    time.tm_hour = to_nonnegative_int(hour(), 24);
     format_localized(time, "%OH");
   }
 
@@ -635,7 +653,7 @@ struct chrono_formatter {
 
     if (ns == numeric_system::standard) return write(hour12(), 2);
     auto time = tm();
-    time.tm_hour = to_int(hour12());
+    time.tm_hour = to_nonnegative_int(hour12(), 12);
     format_localized(time, "%OI");
   }
 
@@ -646,7 +664,7 @@ struct chrono_formatter {
 
     if (ns == numeric_system::standard) return write(minute(), 2);
     auto time = tm();
-    time.tm_min = to_int(minute());
+    time.tm_min = to_nonnegative_int(minute(), 60);
     format_localized(time, "%OM");
   }
 
@@ -679,7 +697,7 @@ struct chrono_formatter {
       return;
     }
     auto time = tm();
-    time.tm_sec = to_int(second());
+    time.tm_sec = to_nonnegative_int(second(), 60);
     format_localized(time, "%OS");
   }
 
diff --git a/include/fmt/format.h b/include/fmt/format.h
index 2dd51bd8ed1e..36498224454f 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -33,6 +33,7 @@
 #include <cmath>
 #include <cstdint>
 #include <cstring>
+#include <iterator>
 #include <limits>
 #include <memory>
 #include <stdexcept>
@@ -94,10 +95,6 @@
 #  endif
 #endif
 
-#if FMT_SECURE_SCL
-#  include <iterator>
-#endif
-
 #ifdef __has_builtin
 #  define FMT_HAS_BUILTIN(x) __has_builtin(x)
 #else
@@ -268,32 +265,9 @@ inline Dest bit_cast(const Source& source) {
   return dest;
 }
 
-// An implementation of begin and end for pre-C++11 compilers such as gcc 4.
-template <typename C>
-FMT_CONSTEXPR auto begin(const C& c) -> decltype(c.begin()) {
-  return c.begin();
-}
-template <typename T, std::size_t N>
-FMT_CONSTEXPR T* begin(T (&array)[N]) FMT_NOEXCEPT {
-  return array;
-}
-template <typename C> FMT_CONSTEXPR auto end(const C& c) -> decltype(c.end()) {
-  return c.end();
-}
-template <typename T, std::size_t N>
-FMT_CONSTEXPR T* end(T (&array)[N]) FMT_NOEXCEPT {
-  return array + N;
-}
-
-// An implementation of iterator_t for pre-C++20 compilers such as gcc 4.
-template <typename T> struct iterator_t {
-  typedef decltype(internal::begin(std::declval<const T&>())) type;
-};
-
-// For std::result_of in gcc 4.4.
-template <typename Result> struct function {
-  template <typename T> struct result { typedef Result type; };
-};
+// An implementation of iterator_t for pre-C++20 systems.
+template <typename T>
+using iterator_t = decltype(std::begin(std::declval<T&>()));
 
 template <typename Allocator>
 typename Allocator::value_type* allocate(Allocator& alloc, std::size_t n) {
@@ -1545,7 +1519,7 @@ FMT_CONSTEXPR unsigned parse_nonnegative_int(const Char*& begin,
   return value;
 }
 
-template <typename Context> class custom_formatter : public function<bool> {
+template <typename Context> class custom_formatter {
  private:
   typedef typename Context::char_type char_type;
 
@@ -1572,8 +1546,7 @@ template <typename T> struct is_integer {
   };
 };
 
-template <typename ErrorHandler>
-class width_checker : public function<unsigned long long> {
+template <typename ErrorHandler> class width_checker {
  public:
   explicit FMT_CONSTEXPR width_checker(ErrorHandler& eh) : handler_(eh) {}
 
@@ -1593,8 +1566,7 @@ class width_checker : public function<unsigned long long> {
   ErrorHandler& handler_;
 };
 
-template <typename ErrorHandler>
-class precision_checker : public function<unsigned long long> {
+template <typename ErrorHandler> class precision_checker {
  public:
   explicit FMT_CONSTEXPR precision_checker(ErrorHandler& eh) : handler_(eh) {}
 
@@ -3351,15 +3323,15 @@ arg_join<It, wchar_t> join(It begin, It end, wstring_view sep) {
   \endrst
  */
 template <typename Range>
-arg_join<typename internal::iterator_t<Range>::type, char> join(
+arg_join<internal::iterator_t<const Range>, char> join(
     const Range& range, string_view sep) {
-  return join(internal::begin(range), internal::end(range), sep);
+  return join(std::begin(range), std::end(range), sep);
 }
 
 template <typename Range>
-arg_join<typename internal::iterator_t<Range>::type, wchar_t> join(
+arg_join<internal::iterator_t<const Range>, wchar_t> join(
     const Range& range, wstring_view sep) {
-  return join(internal::begin(range), internal::end(range), sep);
+  return join(std::begin(range), std::end(range), sep);
 }
 #endif
 
diff --git a/include/fmt/printf.h b/include/fmt/printf.h
index c3a48ed4794e..d52f849c7600 100644
--- a/include/fmt/printf.h
+++ b/include/fmt/printf.h
@@ -38,7 +38,7 @@ template <> struct int_checker<true> {
   static bool fits_in_int(int) { return true; }
 };
 
-class printf_precision_handler : public function<int> {
+class printf_precision_handler {
  public:
   template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
   int operator()(T value) {
@@ -55,7 +55,7 @@ class printf_precision_handler : public function<int> {
 };
 
 // An argument visitor that returns true iff arg is a zero integer.
-class is_zero_int : public function<bool> {
+class is_zero_int {
  public:
   template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
   bool operator()(T value) {
@@ -72,8 +72,7 @@ template <typename T> struct make_unsigned_or_bool : std::make_unsigned<T> {};
 
 template <> struct make_unsigned_or_bool<bool> { typedef bool type; };
 
-template <typename T, typename Context>
-class arg_converter : public function<void> {
+template <typename T, typename Context> class arg_converter {
  private:
   typedef typename Context::char_type Char;
 
@@ -129,7 +128,7 @@ void convert_arg(basic_format_arg<Context>& arg, Char type) {
 }
 
 // Converts an integer argument to char for printf.
-template <typename Context> class char_converter : public function<void> {
+template <typename Context> class char_converter {
  private:
   basic_format_arg<Context>& arg_;
 
@@ -148,8 +147,7 @@ template <typename Context> class char_converter : public function<void> {
 
 // Checks if an argument is a valid printf width specifier and sets
 // left alignment if it is negative.
-template <typename Char>
-class printf_width_handler : public function<unsigned> {
+template <typename Char> class printf_width_handler {
  private:
   typedef basic_format_specs<Char> format_specs;
 
diff --git a/test/format b/test/format
index 53a7b5df5c14..60dc60e56a49 100644
--- a/test/format
+++ b/test/format
@@ -489,9 +489,7 @@ namespace detail {
 
 template <typename Range>
 class arg_formatter
-    : public fmt::internal::function<
-          typename fmt::internal::arg_formatter_base<Range, error_handler>::iterator>,
-      public fmt::internal::arg_formatter_base<Range, error_handler> {
+    : public fmt::internal::arg_formatter_base<Range, error_handler> {
  private:
   using char_type = typename Range::value_type;
   using base = fmt::internal::arg_formatter_base<Range, error_handler>;
diff --git a/test/format-impl-test.cc b/test/format-impl-test.cc
index 94bc5eeafbbf..2b0067b0f850 100644
--- a/test/format-impl-test.cc
+++ b/test/format-impl-test.cc
@@ -143,7 +143,7 @@ TEST(FPTest, GrisuFormatCompilesWithNonIEEEDouble) {
   grisu_format(4.2f, buf, -1, false, exp);
 }
 
-template <typename T> struct ValueExtractor : fmt::internal::function<T> {
+template <typename T> struct value_extractor {
   T operator()(T value) { return value; }
 
   template <typename U> FMT_NORETURN T operator()(U) {
@@ -157,7 +157,7 @@ TEST(FormatTest, ArgConverter) {
   fmt::visit_format_arg(
       fmt::internal::arg_converter<long long, fmt::format_context>(arg, 'd'),
       arg);
-  EXPECT_EQ(value, fmt::visit_format_arg(ValueExtractor<long long>(), arg));
+  EXPECT_EQ(value, fmt::visit_format_arg(value_extractor<long long>(), arg));
 }
 
 TEST(FormatTest, FormatNegativeNaN) {
diff --git a/test/format-test.cc b/test/format-test.cc
index 0d7153e36da8..6c05854cd289 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -1890,9 +1890,7 @@ TEST(FormatTest, FixedEnum) { EXPECT_EQ("0", fmt::format("{}", B)); }
 typedef fmt::back_insert_range<fmt::internal::buffer<char>> buffer_range;
 
 class mock_arg_formatter
-    : public fmt::internal::function<
-          fmt::internal::arg_formatter_base<buffer_range>::iterator>,
-      public fmt::internal::arg_formatter_base<buffer_range> {
+    : public fmt::internal::arg_formatter_base<buffer_range> {
  private:
   MOCK_METHOD1(call, void(long long value));