diff --git a/stl/inc/chrono b/stl/inc/chrono
index 8405b26be5..b19aa83f7c 100644
--- a/stl/inc/chrono
+++ b/stl/inc/chrono
@@ -10,6 +10,7 @@
 #if _STL_COMPILER_PREPROCESSOR
 #include <limits>
 #include <ratio>
+#include <sstream>
 #include <time.h>
 #include <utility>
 #include <xtimec.h>
@@ -617,6 +618,88 @@ namespace chrono {
     };
 
     using high_resolution_clock = steady_clock;
+
+#if _HAS_CXX20
+    // [time.duration.io]
+
+#define _IF_PERIOD_RETURN_SUFFIX_ELSE(_TYPE, _SUFFIX) \
+    if constexpr (is_same_v<_Period, _TYPE>) {        \
+        if constexpr (is_same_v<_CharT, char>) {      \
+            return _SUFFIX;                           \
+        } else {                                      \
+            return L##_SUFFIX;                        \
+        }                                             \
+    } else
+
+    template <class _CharT, class _Period>
+    _NODISCARD constexpr const _CharT* _Get_literal_unit_suffix() {
+        _IF_PERIOD_RETURN_SUFFIX_ELSE(atto, "as")
+        _IF_PERIOD_RETURN_SUFFIX_ELSE(femto, "fs")
+        _IF_PERIOD_RETURN_SUFFIX_ELSE(pico, "ps")
+        _IF_PERIOD_RETURN_SUFFIX_ELSE(nano, "ns")
+        _IF_PERIOD_RETURN_SUFFIX_ELSE(micro, "us")
+        _IF_PERIOD_RETURN_SUFFIX_ELSE(milli, "ms")
+        _IF_PERIOD_RETURN_SUFFIX_ELSE(centi, "cs")
+        _IF_PERIOD_RETURN_SUFFIX_ELSE(deci, "ds")
+        _IF_PERIOD_RETURN_SUFFIX_ELSE(seconds::period, "s")
+        _IF_PERIOD_RETURN_SUFFIX_ELSE(deca, "das")
+        _IF_PERIOD_RETURN_SUFFIX_ELSE(hecto, "hs")
+        _IF_PERIOD_RETURN_SUFFIX_ELSE(kilo, "ks")
+        _IF_PERIOD_RETURN_SUFFIX_ELSE(mega, "Ms")
+        _IF_PERIOD_RETURN_SUFFIX_ELSE(giga, "Gs")
+        _IF_PERIOD_RETURN_SUFFIX_ELSE(tera, "Ts")
+        _IF_PERIOD_RETURN_SUFFIX_ELSE(peta, "Ps")
+        _IF_PERIOD_RETURN_SUFFIX_ELSE(exa, "Es")
+        _IF_PERIOD_RETURN_SUFFIX_ELSE(minutes::period, "min")
+        _IF_PERIOD_RETURN_SUFFIX_ELSE(hours::period, "h")
+        _IF_PERIOD_RETURN_SUFFIX_ELSE(ratio<86400>, "d")
+
+        {
+            return nullptr;
+        }
+    }
+
+#undef _IF_PERIOD_RETURN_SUFFIX_ELSE
+
+    template <class _CharT>
+    _NODISCARD _CharT* _Get_general_unit_suffix(_CharT* _Rnext, const intmax_t _Num, const intmax_t _Den) {
+        // Returns the head pointer of the string, built in reverse.
+        _STL_INTERNAL_CHECK(_Num > 0 && _Den > 0);
+        *--_Rnext = '\0';
+        *--_Rnext = 's';
+        *--_Rnext = ']';
+        if (_Den != 1) {
+            _Rnext    = _UIntegral_to_buff(_Rnext, static_cast<uintmax_t>(_Den));
+            *--_Rnext = '/';
+        }
+
+        _Rnext    = _UIntegral_to_buff(_Rnext, static_cast<uintmax_t>(_Num));
+        *--_Rnext = '[';
+        return _Rnext;
+    }
+
+    template <class _CharT, class _Traits, class _Rep, class _Period>
+    basic_ostream<_CharT, _Traits>& operator<<(
+        basic_ostream<_CharT, _Traits>& _Os, const duration<_Rep, _Period>& _Dur) {
+        basic_ostringstream<_CharT, _Traits> _Sstr;
+        _Sstr.flags(_Os.flags());
+        _Sstr.imbue(_Os.getloc());
+        _Sstr.precision(_Os.precision());
+        _Sstr << _Dur.count();
+
+        constexpr auto _Suffix = _Get_literal_unit_suffix<_CharT, _Period>();
+        if constexpr (_Suffix == nullptr) {
+            _CharT _Buffer[2 * (numeric_limits<intmax_t>::digits10 + 1) + 5] = {}; // 2 numbers + "[/]s\0"
+            const _CharT* const _Begin =
+                _Get_general_unit_suffix<_CharT>(_STD end(_Buffer), _Period::num, _Period::den);
+            _Sstr << _Begin;
+        } else {
+            _Sstr << _Suffix;
+        }
+
+        return _Os << _Sstr.str();
+    }
+#endif // _HAS_CXX20
 } // namespace chrono
 
 // HELPERS
diff --git a/stl/inc/string b/stl/inc/string
index 09802c114e..72d3cebb2d 100644
--- a/stl/inc/string
+++ b/stl/inc/string
@@ -448,7 +448,7 @@ inline long double stold(const wstring& _Str, size_t* _Idx = nullptr) { // conve
     return _Ans;
 }
 
-// HELPERS FOR to_string AND to_wstring
+// HELPERS FOR to_string AND to_wstring AND operator<<(duration)
 template <class _Elem, class _UTy>
 _Elem* _UIntegral_to_buff(_Elem* _RNext, _UTy _UVal) { // format _UVal into buffer *ending at* _RNext
     static_assert(is_unsigned_v<_UTy>, "_UTy must be unsigned");
diff --git a/tests/std/test.lst b/tests/std/test.lst
index adf5a62423..4140f5fd7c 100644
--- a/tests/std/test.lst
+++ b/tests/std/test.lst
@@ -222,6 +222,7 @@ tests\P0220R1_searchers
 tests\P0220R1_string_view
 tests\P0325R4_to_array
 tests\P0339R6_polymorphic_allocator
+tests\P0355R7_calendars_and_time_zones_io
 tests\P0356R5_bind_front
 tests\P0357R3_supporting_incomplete_types_in_reference_wrapper
 tests\P0414R2_shared_ptr_for_arrays
diff --git a/tests/std/tests/P0355R7_calendars_and_time_zones_io/env.lst b/tests/std/tests/P0355R7_calendars_and_time_zones_io/env.lst
new file mode 100644
index 0000000000..642f530ffa
--- /dev/null
+++ b/tests/std/tests/P0355R7_calendars_and_time_zones_io/env.lst
@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\usual_latest_matrix.lst
diff --git a/tests/std/tests/P0355R7_calendars_and_time_zones_io/test.cpp b/tests/std/tests/P0355R7_calendars_and_time_zones_io/test.cpp
new file mode 100644
index 0000000000..fefbba6929
--- /dev/null
+++ b/tests/std/tests/P0355R7_calendars_and_time_zones_io/test.cpp
@@ -0,0 +1,125 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <cassert>
+#include <chrono>
+#include <climits>
+#include <locale>
+#include <ratio>
+#include <sstream>
+#include <string>
+#include <utility>
+
+using namespace std;
+using chrono::duration;
+
+template <class CharT, class Rep, class Period>
+bool test_duration_basic_out(const duration<Rep, Period>& d, const CharT* expected) {
+    basic_ostringstream<CharT> ss;
+
+    ss << d;
+    return ss.str() == expected;
+}
+
+#define WIDEN(TYPE, STR) get<const TYPE*>(make_pair(STR, L##STR));
+
+template <class CharT>
+bool test_duration_locale_out() {
+    basic_stringstream<CharT> ss;
+    const duration<double> d{0.140625};
+    ss.precision(3);
+    ss << d;
+    ss.setf(ios_base::scientific, ios_base::floatfield);
+    ss << ' ' << d;
+
+    basic_string<CharT> expected = WIDEN(CharT, "0.141s 1.406e-01s");
+
+#ifdef _DEBUG
+#define DEFAULT_IDL_SETTING 2
+#else
+#define DEFAULT_IDL_SETTING 0
+#endif
+
+#if !defined(_DLL) || _ITERATOR_DEBUG_LEVEL == DEFAULT_IDL_SETTING
+    // When linking dynamically, user-defined facets are incompatible with non-default _ITERATOR_DEBUG_LEVEL settings.
+    struct comma : numpunct<CharT> {
+        CharT do_decimal_point() const {
+            return ',';
+        }
+    };
+
+    ss.imbue(locale(ss.getloc(), new comma));
+    ss << ' ' << d;
+    expected += WIDEN(CharT, " 1,406e-01s");
+#endif // !defined(_DLL) || _ITERATOR_DEBUG_LEVEL == DEFAULT_IDL_SETTING
+
+    return ss.str() == expected;
+}
+
+#undef WIDEN
+
+void test_duration_output() {
+    using LongRatio = ratio<INTMAX_MAX - 1, INTMAX_MAX>;
+    assert(test_duration_basic_out(duration<int, atto>{1}, "1as"));
+    assert(test_duration_basic_out(duration<int, femto>{2}, "2fs"));
+    assert(test_duration_basic_out(duration<int, pico>{3}, "3ps"));
+    assert(test_duration_basic_out(duration<int, nano>{42}, "42ns"));
+    assert(test_duration_basic_out(duration<int, micro>{42}, "42us"));
+    assert(test_duration_basic_out(duration<int, milli>{42}, "42ms"));
+    assert(test_duration_basic_out(duration<int, centi>{42}, "42cs"));
+    assert(test_duration_basic_out(duration<int, deci>{42}, "42ds"));
+    assert(test_duration_basic_out(duration<int, ratio<1>>{42}, "42s"));
+    assert(test_duration_basic_out(duration<int, deca>{42}, "42das"));
+    assert(test_duration_basic_out(duration<int, hecto>{42}, "42hs"));
+    assert(test_duration_basic_out(duration<int, kilo>{42}, "42ks"));
+    assert(test_duration_basic_out(duration<int, mega>{42}, "42Ms"));
+    assert(test_duration_basic_out(duration<int, giga>{42}, "42Gs"));
+    assert(test_duration_basic_out(duration<int, tera>{42}, "42Ts"));
+    assert(test_duration_basic_out(duration<int, peta>{42}, "42Ps"));
+    assert(test_duration_basic_out(duration<int, exa>{42}, "42Es"));
+    assert(test_duration_basic_out(duration<int, ratio<60>>{42}, "42min"));
+    assert(test_duration_basic_out(duration<int, ratio<60 * 60>>{42}, "42h"));
+    assert(test_duration_basic_out(duration<int, ratio<60 * 60 * 24>>{42}, "42d"));
+
+    assert(test_duration_basic_out(duration<int, ratio<2>>{24}, "24[2]s"));
+    assert(test_duration_basic_out(duration<int, ratio<1, 2>>{24}, "24[1/2]s"));
+    assert(test_duration_basic_out(duration<int, ratio<22, 7>>{24}, "24[22/7]s"));
+    assert(test_duration_basic_out(duration<int, LongRatio>{24}, "24[9223372036854775806/9223372036854775807]s"));
+
+    assert(test_duration_basic_out(duration<double>{0.140625}, "0.140625s"));
+    assert(test_duration_locale_out<char>());
+
+    assert(test_duration_basic_out(duration<int, atto>{1}, L"1as"));
+    assert(test_duration_basic_out(duration<int, femto>{2}, L"2fs"));
+    assert(test_duration_basic_out(duration<int, pico>{3}, L"3ps"));
+    assert(test_duration_basic_out(duration<int, nano>{42}, L"42ns"));
+    assert(test_duration_basic_out(duration<int, micro>{42}, L"42us"));
+    assert(test_duration_basic_out(duration<int, milli>{42}, "42ms"));
+    assert(test_duration_basic_out(duration<int, centi>{42}, L"42cs"));
+    assert(test_duration_basic_out(duration<int, deci>{42}, L"42ds"));
+    assert(test_duration_basic_out(duration<int, ratio<1>>{42}, L"42s"));
+    assert(test_duration_basic_out(duration<int, deca>{42}, L"42das"));
+    assert(test_duration_basic_out(duration<int, hecto>{42}, L"42hs"));
+    assert(test_duration_basic_out(duration<int, kilo>{42}, L"42ks"));
+    assert(test_duration_basic_out(duration<int, mega>{42}, L"42Ms"));
+    assert(test_duration_basic_out(duration<int, giga>{42}, L"42Gs"));
+    assert(test_duration_basic_out(duration<int, tera>{42}, L"42Ts"));
+    assert(test_duration_basic_out(duration<int, peta>{42}, L"42Ps"));
+    assert(test_duration_basic_out(duration<int, exa>{42}, L"42Es"));
+    assert(test_duration_basic_out(duration<int, ratio<60>>{42}, L"42min"));
+    assert(test_duration_basic_out(duration<int, ratio<60 * 60>>{42}, L"42h"));
+    assert(test_duration_basic_out(duration<int, ratio<60 * 60 * 24>>{42}, L"42d"));
+
+    assert(test_duration_basic_out(duration<int, ratio<2>>{24}, L"24[2]s"));
+    assert(test_duration_basic_out(duration<int, ratio<1, 2>>{24}, L"24[1/2]s"));
+    assert(test_duration_basic_out(duration<int, ratio<22, 7>>{24}, L"24[22/7]s"));
+    assert(test_duration_basic_out(duration<int, LongRatio>{24}, L"24[9223372036854775806/9223372036854775807]s"));
+
+    assert(test_duration_basic_out(duration<double>{0.140625}, L"0.140625s"));
+    assert(test_duration_locale_out<wchar_t>());
+}
+
+int main() {
+    test_duration_output();
+    return 0;
+}