diff --git a/datetime/_date_time_formatter.ts b/datetime/_date_time_formatter.ts
index 377b7a3a216e..d7e08b69b5d2 100644
--- a/datetime/_date_time_formatter.ts
+++ b/datetime/_date_time_formatter.ts
@@ -325,17 +325,19 @@ export class DateTimeFormatter {
 
     for (const part of this.#formatParts) {
       const type = part.type;
-
+      let length = 0;
       let value = "";
       switch (part.type) {
         case "year": {
           switch (part.value) {
             case "numeric": {
               value = /^\d{1,4}/.exec(string)?.[0] as string;
+              length = value?.length;
               break;
             }
             case "2-digit": {
               value = /^\d{1,2}/.exec(string)?.[0] as string;
+              length = value?.length;
               break;
             }
             default:
@@ -349,22 +351,27 @@ export class DateTimeFormatter {
           switch (part.value) {
             case "numeric": {
               value = /^\d{1,2}/.exec(string)?.[0] as string;
+              length = value?.length;
               break;
             }
             case "2-digit": {
               value = /^\d{2}/.exec(string)?.[0] as string;
+              length = value?.length;
               break;
             }
             case "narrow": {
               value = /^[a-zA-Z]+/.exec(string)?.[0] as string;
+              length = value?.length;
               break;
             }
             case "short": {
               value = /^[a-zA-Z]+/.exec(string)?.[0] as string;
+              length = value?.length;
               break;
             }
             case "long": {
               value = /^[a-zA-Z]+/.exec(string)?.[0] as string;
+              length = value?.length;
               break;
             }
             default:
@@ -378,10 +385,12 @@ export class DateTimeFormatter {
           switch (part.value) {
             case "numeric": {
               value = /^\d{1,2}/.exec(string)?.[0] as string;
+              length = value?.length;
               break;
             }
             case "2-digit": {
               value = /^\d{2}/.exec(string)?.[0] as string;
+              length = value?.length;
               break;
             }
             default:
@@ -395,6 +404,7 @@ export class DateTimeFormatter {
           switch (part.value) {
             case "numeric": {
               value = /^\d{1,2}/.exec(string)?.[0] as string;
+              length = value?.length;
               if (part.hour12 && parseInt(value) > 12) {
                 // TODO(iuioiua): Replace with throwing an error
                 // deno-lint-ignore no-console
@@ -406,6 +416,7 @@ export class DateTimeFormatter {
             }
             case "2-digit": {
               value = /^\d{2}/.exec(string)?.[0] as string;
+              length = value?.length;
               if (part.hour12 && parseInt(value) > 12) {
                 // TODO(iuioiua): Replace with throwing an error
                 // deno-lint-ignore no-console
@@ -427,10 +438,12 @@ export class DateTimeFormatter {
           switch (part.value) {
             case "numeric": {
               value = /^\d{1,2}/.exec(string)?.[0] as string;
+              length = value?.length;
               break;
             }
             case "2-digit": {
               value = /^\d{2}/.exec(string)?.[0] as string;
+              length = value?.length;
               break;
             }
             default:
@@ -444,10 +457,12 @@ export class DateTimeFormatter {
           switch (part.value) {
             case "numeric": {
               value = /^\d{1,2}/.exec(string)?.[0] as string;
+              length = value?.length;
               break;
             }
             case "2-digit": {
               value = /^\d{2}/.exec(string)?.[0] as string;
+              length = value?.length;
               break;
             }
             default:
@@ -460,24 +475,40 @@ export class DateTimeFormatter {
         case "fractionalSecond": {
           value = new RegExp(`^\\d{${part.value}}`).exec(string)
             ?.[0] as string;
+          length = value?.length;
           break;
         }
         case "timeZoneName": {
           value = part.value as string;
+          length = value?.length;
           break;
         }
         case "dayPeriod": {
           value = /^[AP](?:\.M\.|M\.?)/i.exec(string)?.[0] as string;
           switch (value.toUpperCase()) {
             case "AM":
+              value = "AM";
+              length = 2;
+              break;
             case "AM.":
+              value = "AM";
+              length = 3;
+              break;
             case "A.M.":
               value = "AM";
+              length = 4;
               break;
             case "PM":
+              value = "PM";
+              length = 2;
+              break;
             case "PM.":
+              value = "PM";
+              length = 3;
+              break;
             case "P.M.":
               value = "PM";
+              length = 4;
               break;
             default:
               throw new Error(`DayPeriod '${value}' is not supported.`);
@@ -491,6 +522,7 @@ export class DateTimeFormatter {
             );
           }
           value = part.value as string;
+          length = value?.length;
           break;
         }
 
@@ -512,7 +544,7 @@ export class DateTimeFormatter {
       }
       parts.push({ type, value });
 
-      string = string.slice(value.length);
+      string = string.slice(length);
     }
 
     if (string.length) {
diff --git a/datetime/_date_time_formatter_test.ts b/datetime/_date_time_formatter_test.ts
index c4d9167b8240..e7c450336d04 100644
--- a/datetime/_date_time_formatter_test.ts
+++ b/datetime/_date_time_formatter_test.ts
@@ -43,16 +43,208 @@ Deno.test("dateTimeFormatter.parse()", () => {
   assertEquals(formatter.parse("2020-01-01"), new Date(2020, 0, 1));
 });
 
-Deno.test("dateTimeFormatter.formatToParts()", () => {
-  const format = "yyyy-MM-dd";
-  const formatter = new DateTimeFormatter(format);
-  assertEquals(formatter.formatToParts("2020-01-01"), [
-    { type: "year", value: "2020" },
-    { type: "literal", value: "-" },
-    { type: "month", value: "01" },
-    { type: "literal", value: "-" },
-    { type: "day", value: "01" },
-  ]);
+Deno.test("dateTimeFormatter.formatToParts()", async (t) => {
+  await t.step("handles basic", () => {
+    const format = "yyyy-MM-dd";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("2020-01-01"), [
+      { type: "year", value: "2020" },
+      { type: "literal", value: "-" },
+      { type: "month", value: "01" },
+      { type: "literal", value: "-" },
+      { type: "day", value: "01" },
+    ]);
+  });
+
+  await t.step("handles yy", () => {
+    const format = "yy";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("20"), [
+      { type: "year", value: "20" },
+    ]);
+  });
+  await t.step("handles yyyy", () => {
+    const format = "yyyy";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("2020"), [
+      { type: "year", value: "2020" },
+    ]);
+  });
+  await t.step("handles M", () => {
+    const format = "M";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("10"), [
+      { type: "month", value: "10" },
+    ]);
+  });
+  await t.step("handles MM", () => {
+    const format = "MM";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("10"), [
+      { type: "month", value: "10" },
+    ]);
+  });
+  await t.step("handles d", () => {
+    const format = "d";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("10"), [
+      { type: "day", value: "10" },
+    ]);
+  });
+  await t.step("handles dd", () => {
+    const format = "dd";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("10"), [
+      { type: "day", value: "10" },
+    ]);
+  });
+  await t.step("handles h", () => {
+    const format = "h";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("1"), [
+      { type: "hour", value: "1" },
+    ]);
+  });
+  await t.step("handles hh", () => {
+    const format = "hh";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("11"), [
+      { type: "hour", value: "11" },
+    ]);
+  });
+  await t.step("handles h value bigger than 12 warning", () => {
+    const format = "h";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("13"), [
+      { type: "hour", value: "13" },
+    ]);
+  });
+  await t.step("handles hh value bigger than 12 warning", () => {
+    const format = "hh";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("13"), [
+      { type: "hour", value: "13" },
+    ]);
+  });
+  await t.step("handles H", () => {
+    const format = "H";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("13"), [
+      { type: "hour", value: "13" },
+    ]);
+  });
+  await t.step("handles HH", () => {
+    const format = "HH";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("13"), [
+      { type: "hour", value: "13" },
+    ]);
+  });
+  await t.step("handles m", () => {
+    const format = "m";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("10"), [
+      { type: "minute", value: "10" },
+    ]);
+  });
+  await t.step("handles mm", () => {
+    const format = "mm";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("10"), [
+      { type: "minute", value: "10" },
+    ]);
+  });
+  await t.step("handles s", () => {
+    const format = "s";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("10"), [
+      { type: "second", value: "10" },
+    ]);
+  });
+  await t.step("handles ss", () => {
+    const format = "ss";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("10"), [
+      { type: "second", value: "10" },
+    ]);
+  });
+  await t.step("handles s", () => {
+    const format = "s";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("10"), [
+      { type: "second", value: "10" },
+    ]);
+  });
+  await t.step("handles S", () => {
+    const format = "S";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("1"), [
+      { type: "fractionalSecond", value: "1" },
+    ]);
+  });
+  await t.step("handles SS", () => {
+    const format = "SS";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("10"), [
+      { type: "fractionalSecond", value: "10" },
+    ]);
+  });
+  await t.step("handles SSS", () => {
+    const format = "SSS";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("100"), [
+      { type: "fractionalSecond", value: "100" },
+    ]);
+  });
+  await t.step("handles a", () => {
+    const format = "a";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("AM"), [
+      { type: "dayPeriod", value: "AM" },
+    ]);
+  });
+  await t.step("handles a AM", () => {
+    const format = "a";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("AM"), [
+      { type: "dayPeriod", value: "AM" },
+    ]);
+  });
+  await t.step("handles a AM.", () => {
+    const format = "a";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("AM."), [
+      { type: "dayPeriod", value: "AM" },
+    ]);
+  });
+  await t.step("handles a A.M.", () => {
+    const format = "a";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("A.M."), [
+      { type: "dayPeriod", value: "AM" },
+    ]);
+  });
+  await t.step("handles a PM", () => {
+    const format = "a";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("PM"), [
+      { type: "dayPeriod", value: "PM" },
+    ]);
+  });
+  await t.step("handles a PM.", () => {
+    const format = "a";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("PM."), [
+      { type: "dayPeriod", value: "PM" },
+    ]);
+  });
+  await t.step("handles a P.M.", () => {
+    const format = "a";
+    const formatter = new DateTimeFormatter(format);
+    assertEquals(formatter.formatToParts("P.M."), [
+      { type: "dayPeriod", value: "PM" },
+    ]);
+  });
 });
 
 Deno.test("dateTimeFormatter.formatToParts() throws on an empty string", () => {
@@ -285,6 +477,10 @@ Deno.test("dateTimeFormatter.partsToDate() sets utc", () => {
       { type: "month", value: "01" },
       { type: "timeZoneName", value: "UTC" },
     ], date],
+    ["MM", [
+      { type: "month", value: "01" },
+      { type: "timeZoneName", value: "UTC" },
+    ], date],
   ] as const;
   for (const [format, input, output] of cases) {
     const formatter = new DateTimeFormatter(format);
diff --git a/datetime/parse_test.ts b/datetime/parse_test.ts
index 5c08a59f67ea..45744c18ad44 100644
--- a/datetime/parse_test.ts
+++ b/datetime/parse_test.ts
@@ -38,6 +38,14 @@ Deno.test({
       parse("01-03-2019 09:33 PM", "MM-dd-yyyy HH:mm a"),
       new Date(2019, 0, 3, 21, 33),
     );
+    assertEquals(
+      parse("01-03-2019 09:33 PM.", "MM-dd-yyyy HH:mm a"),
+      new Date(2019, 0, 3, 21, 33),
+    );
+    assertEquals(
+      parse("01-03-2019 09:33 P.M.", "MM-dd-yyyy HH:mm a"),
+      new Date(2019, 0, 3, 21, 33),
+    );
     assertEquals(
       parse("16:34 03-01-2019", "HH:mm dd-MM-yyyy"),
       new Date(2019, 0, 3, 16, 34),