diff --git a/install/supabase/functions/canvasgradebook/handler.js b/install/supabase/functions/canvasgradebook/handler.js
index a6ff15b..905b181 100644
--- a/install/supabase/functions/canvasgradebook/handler.js
+++ b/install/supabase/functions/canvasgradebook/handler.js
@@ -14,13 +14,17 @@ function normalizePercent(percentCorrect) {
 // Canvas collapses plain-text newlines in submission comments, so the header
 // lines are joined with <br>. The feedback is already simplified HTML produced
 // by the client (markdownToHtml), so it is appended as-is rather than escaped.
-export function buildCanvasComment({ feedback, normalizedPercent, normalizedPoints, postedGrade, autoGrade }) {
+export function buildCanvasComment({ feedback, normalizedPercent, normalizedPoints, postedGrade, autoGrade, dateDue }) {
   const lines = [];
+  const date = new Date().toISOString();
   const suggestedGrade = Math.round(((normalizedPercent / 100) * normalizedPoints + Number.EPSILON) * 100) / 100;
   lines.push('MasteryLS feedback');
   lines.push(`Suggested grade: ${suggestedGrade}/${normalizedPoints} (${normalizedPercent}%)`);
   lines.push(`Auto grade: ${autoGrade ? 'enabled' : 'disabled'}`);
-  lines.push(`Submitted at: ${new Date().toISOString()}`);
+  lines.push(`Submitted at: ${date}`);
+  if (dateDue) {
+    lines.push(`Grace Day Potential: ${calculateGraceDaysEarned({ dateSubmitted: date, dateDue })}`);
+  }
   if (typeof postedGrade === 'number') {
     lines.push(`Posted grade: ${postedGrade}`);
   }
@@ -35,6 +39,28 @@ export function buildCanvasComment({ feedback, normalizedPercent, normalizedPoin
   return comment;
 }
 
+export function calculateGraceDaysEarned({ dateSubmitted, dateDue }) {
+  const submitted = new Date(dateSubmitted);
+  const due = new Date(dateDue);
+  const msPerDay = 1000 * 60 * 60 * 24;
+  const isLate = submitted > due;
+  const round = isLate ? Math.ceil : Math.floor;
+  const direction = isLate ? -1 : 1;
+  let graceDaysEarned = 0;
+  let currentDate = new Date(submitted);
+  while (currentDate.toDateString() !== due.toDateString()) {
+    if(currentDate.getDay() === 6) {
+      currentDate = new Date(currentDate.getTime() + direction * msPerDay);
+      continue;
+    }
+    graceDaysEarned += direction;
+    currentDate = new Date(currentDate.getTime() + direction * msPerDay);
+  }
+
+  if( graceDaysEarned === 0 && isLate ) graceDaysEarned = -1;
+  return graceDaysEarned;
+}
+
 export function createCanvasGradebookHandler({ createSupabaseClientFromAuthHeader, getEnv, fetchFn = fetch }) {
   return async function handleCanvasGradebook(req) {
     if (req.method === 'OPTIONS') {
@@ -213,6 +239,13 @@ export function createCanvasGradebookHandler({ createSupabaseClientFromAuthHeade
         return new Response(JSON.stringify({ error: 'Unable to resolve Canvas assignment id' }), { status: 400, headers: { ...corsHeaders, 'Content-Type': 'application/json' } });
       }
 
+      // Mastery updates never attach a comment, so there's no need to look up the due date.
+      let dateDue = null;
+      if (!isMastery) {
+        const assignment = await canvasApi(`/courses/${courseId}/assignments/${assignmentId}`);
+        dateDue = assignment?.due_at || null;
+      }
+
       if (submissionUrl) {
         await canvasApi(`/courses/${courseId}/assignments/${assignmentId}/submissions`, 'POST', {
           submission: {
@@ -248,7 +281,7 @@ export function createCanvasGradebookHandler({ createSupabaseClientFromAuthHeade
           ? {}
           : {
               comment: {
-                text_comment: buildCanvasComment({ feedback, normalizedPercent, normalizedPoints, postedGrade, autoGrade }),
+                text_comment: buildCanvasComment({ feedback, normalizedPercent, normalizedPoints, postedGrade, autoGrade, dateDue }),
               },
             }),
       };
diff --git a/install/supabase/functions/canvasgradebook/handler.test.js b/install/supabase/functions/canvasgradebook/handler.test.js
index c7998e9..eda2372 100644
--- a/install/supabase/functions/canvasgradebook/handler.test.js
+++ b/install/supabase/functions/canvasgradebook/handler.test.js
@@ -1,6 +1,6 @@
 import test from 'node:test';
 import assert from 'node:assert/strict';
-import { createCanvasGradebookHandler, buildCanvasComment } from './handler.js';
+import { createCanvasGradebookHandler, buildCanvasComment, calculateGraceDaysEarned } from './handler.js';
 
 function createMockSupabase({ user, roles }) {
   return {
@@ -48,6 +48,9 @@ function buildFetchStub() {
     if (url.includes('/quizzes/')) {
       return new Response(JSON.stringify({ assignment_id: 555 }), { status: 200 });
     }
+    if (url.includes('/assignments/') && !url.includes('/submissions') && method === 'GET') {
+      return new Response(JSON.stringify({ id: 999, due_at: '2026-05-10T23:59:00Z' }), { status: 200 });
+    }
     if (url.includes('/submissions') && method === 'POST') {
       return new Response(JSON.stringify({ id: 321, submission_type: 'online_url' }), { status: 200 });
     }
@@ -74,6 +77,51 @@ function getSubmissionRequest(calls, method = 'PUT') {
   return JSON.parse(call.init.body || '{}');
 }
 
+function graceDayTest({ dateSubmitted, dateDue, expectedGraceDays}) {
+  const result = calculateGraceDaysEarned({ dateSubmitted, dateDue });
+  assert.equal(result, expectedGraceDays);
+}
+
+test('calculateGraceDaysEarned returns 0 for submission on the due date', () => {
+  graceDayTest({
+    dateSubmitted: new Date('2026-09-10T06:00:00Z'), // A thursday
+    dateDue: new Date('2026-09-10T12:00:00Z'), // A thursday
+    expectedGraceDays: 0
+  });
+});
+
+test('calculateGraceDaysEarned returns -1 for submission slightly after the due date', () => {
+  graceDayTest({
+    dateSubmitted: new Date('2026-09-09T12:00:01Z'), // A wednesday
+    dateDue: new Date('2026-09-09T12:00:00Z'), // A wednesday
+    expectedGraceDays: -1
+  });
+});
+
+test('calculateGraceDaysEarned returns -1 for submission sunday after a saturday due date', () => {
+  graceDayTest({
+    dateSubmitted: new Date('2026-09-13T12:00:00Z'), // A sunday
+    dateDue: new Date('2026-09-12T12:00:00Z'), // A saturday
+    expectedGraceDays: -1
+  });
+});
+
+test('calculateGraceDaysEarned returns -1 for submission sunday after a friday due date', () => {
+  graceDayTest({
+    dateSubmitted: new Date('2026-09-13T12:00:00Z'), // A sunday
+    dateDue: new Date('2026-09-11T12:00:00Z'), // A friday
+    expectedGraceDays: -1
+  });
+});
+
+test('calculateGraceDaysEarned returns 1 for submission saturday before a monday due date', () => {
+  graceDayTest({
+    dateSubmitted: new Date('2026-09-12T12:00:00Z'), // A saturday
+    dateDue: new Date('2026-09-14T12:00:00Z'), // A friday
+    expectedGraceDays: 1
+  });
+});
+
 test('canvasgradebook allows root user', async () => {
   const { fetchFn, calls } = buildFetchStub();
   const handler = createCanvasGradebookHandler({
@@ -99,7 +147,7 @@ test('canvasgradebook allows root user', async () => {
   );
 
   assert.equal(response.status, 200);
-  assert.equal(calls.length, 2);
+  assert.equal(calls.length, 3);
   const submissionRequest = getSubmissionRequest(calls, 'PUT');
   assert.equal(submissionRequest.submission.posted_grade, 90);
   assert.ok(typeof submissionRequest.comment?.text_comment === 'string');
@@ -137,7 +185,7 @@ test('canvasgradebook allows learner self-match', async () => {
   assert.equal(body.submission.url, 'https://example.com/project');
   assert.ok(body.submission.submitted_at);
   assert.notEqual(body.submission.workflow_state, 'unsubmitted');
-  assert.equal(calls.length, 3);
+  assert.equal(calls.length, 4);
   const submissionRequest = getSubmissionRequest(calls, 'PUT');
   assert.equal(submissionRequest.submission.posted_grade, 160);
   assert.ok(submissionRequest.comment.text_comment.includes('Suggested grade: 160/200 (80%)'));
@@ -197,7 +245,7 @@ test('canvasgradebook can submit comment and url without posting grade when auto
   );
 
   assert.equal(response.status, 200);
-  assert.equal(calls.length, 3);
+  assert.equal(calls.length, 4);
 
   const submitAttemptRequest = getSubmissionRequest(calls, 'POST');
   assert.equal(submitAttemptRequest.submission.submission_type, 'online_url');