diff --git a/src/impl/locale.js b/src/impl/locale.js
index d3cb34744..407336b57 100644
--- a/src/impl/locale.js
+++ b/src/impl/locale.js
@@ -75,22 +75,26 @@ function parseLocaleString(localeStr) {
     return [localeStr];
   } else {
     let options;
+    let selectedStr;
     const smaller = localeStr.substring(0, uIndex);
     try {
       options = getCachedDTF(localeStr).resolvedOptions();
+      selectedStr = localeStr;
     } catch (e) {
       options = getCachedDTF(smaller).resolvedOptions();
+      selectedStr = smaller;
     }
 
     const { numberingSystem, calendar } = options;
-    // return the smaller one so that we can append the calendar and numbering overrides to it
-    return [smaller, numberingSystem, calendar];
+    return [selectedStr, numberingSystem, calendar];
   }
 }
 
 function intlConfigString(localeStr, numberingSystem, outputCalendar) {
   if (outputCalendar || numberingSystem) {
-    localeStr += "-u";
+    if (!localeStr.includes("-u-")) {
+      localeStr += "-u";
+    }
 
     if (outputCalendar) {
       localeStr += `-ca-${outputCalendar}`;
diff --git a/test/datetime/create.test.js b/test/datetime/create.test.js
index 446ad9e6f..c14d13e6e 100644
--- a/test/datetime/create.test.js
+++ b/test/datetime/create.test.js
@@ -663,7 +663,7 @@ test("DateTime.fromObject accepts a locale", () => {
 
 test("DateTime.fromObject accepts a locale with calendar and numbering identifiers", () => {
   const res = DateTime.fromObject({}, { locale: "be-u-ca-coptic-nu-mong" });
-  expect(res.locale).toBe("be");
+  expect(res.locale).toBe("be-u-ca-coptic-nu-mong");
   expect(res.outputCalendar).toBe("coptic");
   expect(res.numberingSystem).toBe("mong");
 });
@@ -677,7 +677,7 @@ test("DateTime.fromObject accepts a locale string with weird junk in it", () =>
       }
     );
 
-    expect(res.locale).toBe("be");
+    expect(res.locale).toBe("be-u-ca-coptic-ca-islamic");
 
     // "coptic" is right, but some versions of Node 10 give "gregory"
     expect(res.outputCalendar === "gregory" || res.outputCalendar === "coptic").toBe(true);
@@ -695,7 +695,7 @@ test("DateTime.fromObject overrides the locale string with explicit settings", (
     }
   );
 
-  expect(res.locale).toBe("be");
+  expect(res.locale).toBe("be-u-ca-coptic-nu-mong");
   expect(res.outputCalendar).toBe("islamic");
   expect(res.numberingSystem).toBe("thai");
 });
diff --git a/test/datetime/format.test.js b/test/datetime/format.test.js
index 823dce354..23dcfd77a 100644
--- a/test/datetime/format.test.js
+++ b/test/datetime/format.test.js
@@ -415,6 +415,12 @@ test("DateTime#toLocaleString uses locale-appropriate time formats", () => {
   expect(dt.reconfigure({ locale: "es" }).toLocaleString(DateTime.TIME_24_SIMPLE)).toBe("9:23");
 });
 
+test("DateTime#toLocaleString() respects language tags", () => {
+  expect(dt.reconfigure({ locale: "en-US-u-hc-h23" }).toLocaleString(DateTime.TIME_SIMPLE)).toBe(
+    "09:23"
+  );
+});
+
 test("DateTime#toLocaleString() accepts a zone even when the zone is set", () => {
   expect(
     dt.toLocaleString({