diff --git a/api/prisma/schema.prisma b/api/prisma/schema.prisma
index 93c9ab81ae3..00ad0dd1873 100644
--- a/api/prisma/schema.prisma
+++ b/api/prisma/schema.prisma
@@ -20,7 +20,7 @@ type File {
 
 type CompletedChallenge {
   challengeType      Int?         @db.Int // Null | Undefined
-  completedDate      Float // TODO(Post-MVP): Change to DateTime
+  completedDate      Json // DateTime | Float, but not, as far as we know, Null
   files              File[]
   githubLink         String? // Undefined
   id                 String
diff --git a/api/src/routes/helpers/certificate-utils.ts b/api/src/routes/helpers/certificate-utils.ts
index bb4a3c8c378..01b14f71f7a 100644
--- a/api/src/routes/helpers/certificate-utils.ts
+++ b/api/src/routes/helpers/certificate-utils.ts
@@ -1,7 +1,9 @@
+import { Prisma } from '@prisma/client';
 import {
   certSlugTypeMap,
   certIds
 } from '../../../../shared/config/certification-settings';
+import { normalizeDate } from '../../utils/normalize';
 
 const {
   legacyInfosecQaId,
@@ -41,12 +43,16 @@ export function isKnownCertSlug(
  * @returns The latest certification date or the completed date if no certification is found.
  */
 export function getFallbackFullStackDate(
-  completedChallenges: { id: string; completedDate: number }[],
-  completedDate: number
-) {
+  completedChallenges: { id: string; completedDate: Prisma.JsonValue }[],
+  completedDate: Prisma.JsonValue
+): number {
   const latestCertDate = completedChallenges
     .filter(chal => fullStackCertificateIds.includes(chal.id))
+    .map(chal => ({
+      ...chal,
+      completedDate: normalizeDate(chal.completedDate)
+    }))
     .sort((a, b) => b.completedDate - a.completedDate)[0]?.completedDate;
 
-  return latestCertDate ?? completedDate;
+  return latestCertDate ?? normalizeDate(completedDate);
 }
diff --git a/api/src/routes/protected/challenge.ts b/api/src/routes/protected/challenge.ts
index b4e17d4c549..751cd7a081e 100644
--- a/api/src/routes/protected/challenge.ts
+++ b/api/src/routes/protected/challenge.ts
@@ -35,6 +35,7 @@ import {
   verifyTrophyWithMicrosoft
 } from '../helpers/challenge-helpers';
 import { UpdateReqType } from '../../utils';
+import { normalizeDate } from '../../utils/normalize';
 
 interface JwtPayload {
   userToken: string;
@@ -263,13 +264,12 @@ export const challengeRoutes: FastifyPluginCallbackTypebox = (
           'User tried to submit a codeRoad cert project before completing the required challenges'
         );
         void reply.code(403);
-        return {
+        return reply.send({
           type: 'error',
           message:
             'You have to complete the project before you can submit a URL.'
-        } as const;
+        });
       }
-
       const challenge = {
         challengeType,
         solution,
@@ -287,13 +287,13 @@ export const challengeRoutes: FastifyPluginCallbackTypebox = (
         challenge
       );
 
-      return {
+      reply.send({
         alreadyCompleted,
         // TODO(Post-MVP): audit the client and remove this if the client does
         // not use it.
-        completedDate,
+        completedDate: normalizeDate(completedDate),
         points: alreadyCompleted ? points : points + 1
-      };
+      });
     }
   );
 
@@ -685,11 +685,11 @@ export const challengeRoutes: FastifyPluginCallbackTypebox = (
             completedChallenge
           );
 
-        return {
+        reply.send({
           alreadyCompleted,
           points: getPoints(progressTimestamps) + (alreadyCompleted ? 0 : 1),
-          completedDate
-        };
+          completedDate: normalizeDate(completedDate)
+        });
       } catch (error) {
         logger.error(error, 'Error submitting Microsoft trophy challenge');
         fastify.Sentry.captureException(error);
@@ -808,7 +808,10 @@ export const challengeRoutes: FastifyPluginCallbackTypebox = (
         }
 
         const newCompletedChallenges: CompletedChallenge[] =
-          completedChallenges;
+          completedChallenges.map(c => {
+            const { completedDate, ...rest } = c;
+            return { completedDate: normalizeDate(completedDate), ...rest };
+          });
         const newCompletedExams: CompletedExam[] = completedExams;
         const newProgressTimeStamps = progressTimestamps as ProgressTimestamp[];
         const completedDate = Date.now();
diff --git a/api/src/routes/public/certificate.ts b/api/src/routes/public/certificate.ts
index 4e1360bb95c..f9027ec9d43 100644
--- a/api/src/routes/public/certificate.ts
+++ b/api/src/routes/public/certificate.ts
@@ -13,6 +13,7 @@ import {
   getFallbackFullStackDate,
   isKnownCertSlug
 } from '../helpers/certificate-utils';
+import { normalizeDate } from '../../utils/normalize';
 
 /**
  * Plugin for the unprotected certificate endpoints.
@@ -228,7 +229,7 @@ export const unprotectedCertificateRoutes: FastifyPluginCallbackTypebox = (
           certSlug,
           certTitle,
           username,
-          date: completedDate,
+          date: normalizeDate(completedDate),
           completionTime
         });
       }
@@ -239,7 +240,7 @@ export const unprotectedCertificateRoutes: FastifyPluginCallbackTypebox = (
         certTitle,
         username,
         name,
-        date: completedDate,
+        date: normalizeDate(completedDate),
         completionTime
       });
     }
diff --git a/api/src/utils/common-challenge-functions.ts b/api/src/utils/common-challenge-functions.ts
index a136298a019..2cf46fa0112 100644
--- a/api/src/utils/common-challenge-functions.ts
+++ b/api/src/utils/common-challenge-functions.ts
@@ -3,6 +3,7 @@ import { FastifyInstance } from 'fastify';
 import { omit, pick } from 'lodash';
 import { challengeTypes } from '../../../shared/config/challenge-types';
 import { getChallenges } from './get-challenges';
+import { normalizeDate } from './normalize';
 
 export const jsCertProjectIds = [
   'aaa48de84e1ecc7c742e1124',
@@ -149,7 +150,8 @@ export async function updateUserChallengeData(
             'path',
             'ext'
           ]) as CompletedChallengeFile
-      )
+      ),
+      completedDate: normalizeDate(_completedChallenge.completedDate)
     };
   } else {
     completedChallenge = omit(_completedChallenge, ['files']);
@@ -171,7 +173,7 @@ export async function updateUserChallengeData(
   const finalChallenge = alreadyCompleted
     ? {
         ...completedChallenge,
-        completedDate: oldChallenge.completedDate
+        completedDate: normalizeDate(oldChallenge.completedDate)
       }
     : completedChallenge;
 
@@ -180,7 +182,11 @@ export async function updateUserChallengeData(
   // check and update some property of the user record such that the same update
   // can't be applied twice.
   const userCompletedChallenges = alreadyCompleted
-    ? completedChallenges.map(x => (x.id === challengeId ? finalChallenge : x))
+    ? completedChallenges.map(x =>
+        x.id === challengeId
+          ? finalChallenge
+          : { ...x, completedDate: normalizeDate(x.completedDate) }
+      )
     : { push: finalChallenge };
 
   // We can't use push, because progressTimestamps is a JSON blob and, until
diff --git a/api/src/utils/normalize.test.ts b/api/src/utils/normalize.test.ts
index 144febfa411..94a38321f6e 100644
--- a/api/src/utils/normalize.test.ts
+++ b/api/src/utils/normalize.test.ts
@@ -2,7 +2,8 @@ import {
   normalizeTwitter,
   normalizeProfileUI,
   normalizeChallenges,
-  normalizeFlags
+  normalizeFlags,
+  normalizeDate
 } from './normalize';
 
 describe('normalize', () => {
@@ -156,4 +157,22 @@ describe('normalize', () => {
       });
     });
   });
+
+  describe('validateDate', () => {
+    it('should return the date as a number', () => {
+      expect(normalizeDate(1)).toEqual(1);
+      expect(normalizeDate({ $date: '2023-10-01T00:00:00Z' })).toEqual(
+        1696118400000
+      );
+    });
+
+    it('should throw an error if the date is not in the expected shape', () => {
+      expect(() => normalizeDate('2023-10-01T00:00:00Z')).toThrow(
+        'Unexpected date value: "2023-10-01T00:00:00Z"'
+      );
+      expect(() => normalizeDate({ date: '123' })).toThrow(
+        'Unexpected date value: {"date":"123"}'
+      );
+    });
+  });
 });
diff --git a/api/src/utils/normalize.ts b/api/src/utils/normalize.ts
index bc2df169576..4bd341dcddb 100644
--- a/api/src/utils/normalize.ts
+++ b/api/src/utils/normalize.ts
@@ -1,10 +1,11 @@
 /* This module's job is to parse the database output and prepare it for
 serialization */
-import {
+import type {
   ProfileUI,
   CompletedChallenge,
   ExamResults,
-  type Survey
+  Survey,
+  Prisma
 } from '@prisma/client';
 import _ from 'lodash';
 
@@ -39,6 +40,27 @@ export const normalizeTwitter = (
   return url ?? handleOrUrl;
 };
 
+/**
+ * Normalizes a date value to a timestamp number.
+ *
+ * @param date An object with a $date string or a number.
+ * @returns The date as a timestamp number.
+ */
+export const normalizeDate = (date?: Prisma.JsonValue): number => {
+  if (typeof date === 'number') {
+    return date;
+  } else if (
+    date &&
+    typeof date === 'object' &&
+    '$date' in date &&
+    typeof date.$date === 'string'
+  ) {
+    return new Date(date.$date).getTime();
+  } else {
+    throw Error('Unexpected date value: ' + JSON.stringify(date));
+  }
+};
+
 /**
  * Ensure that the user's profile UI settings are valid.
  *
@@ -114,7 +136,15 @@ export const normalizeChallenges = (
     return { ...rest, files: noNullFiles };
   });
 
-  return noNullPath;
+  const withNumberDates = noNullPath.map(challenge => {
+    const { completedDate, ...rest } = challenge;
+    return {
+      ...rest,
+      completedDate: normalizeDate(completedDate)
+    };
+  });
+
+  return withNumberDates;
 };
 
 type NormalizedSurvey = {