diff --git a/package-lock.json b/package-lock.json
index 898f3f5..b3bd10f 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -6,6 +6,7 @@
 		"": {
 			"name": "smart-shopping-list-next",
 			"dependencies": {
+				"@the-collab-lab/shopping-list-utils": "^2.2.0",
 				"firebase": "^10.1.0",
 				"react": "^18.2.0",
 				"react-dom": "^18.2.0",
@@ -3994,6 +3995,15 @@
 				"@testing-library/dom": ">=7.21.4"
 			}
 		},
+		"node_modules/@the-collab-lab/shopping-list-utils": {
+			"version": "2.2.0",
+			"resolved": "https://registry.npmjs.org/@the-collab-lab/shopping-list-utils/-/shopping-list-utils-2.2.0.tgz",
+			"integrity": "sha512-nEN1z/SEOIWO+8JWIgPDNUkrXmXqNomSh5aiZDNdiaUH/JYqyNeXmEOldtMqAfLicSRiFkapcAIlrUUnPzNaog==",
+			"peerDependencies": {
+				"react": "^18.2.0",
+				"react-dom": "^18.2.0"
+			}
+		},
 		"node_modules/@tootallnate/once": {
 			"version": "2.0.0",
 			"resolved": "https://registry.npmjs.org/@tootallnate/once/-/once-2.0.0.tgz",
diff --git a/package.json b/package.json
index c4a636a..1ac60de 100644
--- a/package.json
+++ b/package.json
@@ -7,6 +7,7 @@
 		"npm": ">=8.19.0"
 	},
 	"dependencies": {
+		"@the-collab-lab/shopping-list-utils": "^2.2.0",
 		"firebase": "^10.1.0",
 		"react": "^18.2.0",
 		"react-dom": "^18.2.0",
diff --git a/src/api/firebase.js b/src/api/firebase.js
index 360664d..41b07b1 100644
--- a/src/api/firebase.js
+++ b/src/api/firebase.js
@@ -11,7 +11,9 @@ import {
 } from 'firebase/firestore';
 import { useEffect, useState } from 'react';
 import { db } from './config';
-import { getFutureDate } from '../utils';
+import { getFutureDate, getDaysBetweenDates } from '../utils';
+import { calculateEstimate } from '@the-collab-lab/shopping-list-utils';
+// calculateEstimate takes 3 arguments - previousEstimate (number), daysSinceLastPurchase (number), totalPurchases (number)
 
 /**
  * A custom hook that subscribes to the user's shopping lists in our Firestore
@@ -179,7 +181,7 @@ export async function addItem(listPath, { itemName, daysUntilNextPurchase }) {
 		dateCreated: new Date(),
 		// NOTE: This is null because the item has just been created.
 		// We'll use updateItem to put a Date here when the item is purchased!
-		dateLastPurchased: null,
+		dateLastPurchased: [new Date()],
 		dateNextPurchased: getFutureDate(daysUntilNextPurchase),
 		name: itemName,
 		totalPurchases: 0,
@@ -187,13 +189,54 @@ export async function addItem(listPath, { itemName, daysUntilNextPurchase }) {
 	return newItem;
 }
 
+//Helper function to handle estimation logic
+async function handleCalculateEstimate(listRef) {
+	const selectedItem = await getDoc(listRef);
+
+	//access array of dateLastPurchased for the selected item
+	const selectedLastPurchase = selectedItem.data().dateLastPurchased;
+
+	//retrieve the most recent purchase date from the array and convert to a date
+	const lastPurchased =
+		selectedLastPurchase[selectedLastPurchase.length - 1].toDate();
+
+	//calculates the previously estimated days until next purchase
+	const previousEstimate = getDaysBetweenDates(
+		lastPurchased,
+		selectedItem.data().dateNextPurchased.toDate(),
+	);
+	const daysSincePrevPurchase = getDaysBetweenDates(lastPurchased, new Date());
+
+	//calculates a new estimate based on the previous estimate, days since last purchased, and total purchases
+	const newEstimate = calculateEstimate(
+		previousEstimate,
+		daysSincePrevPurchase,
+		selectedItem.data().totalPurchases,
+	);
+
+	return { newEstimate, selectedLastPurchase };
+}
+
 export async function updateItem(listPath, itemID, isChecked) {
 	const listRef = doc(db, listPath, 'items', itemID);
 
-	await updateDoc(listRef, {
-		dateLastPurchased: isChecked ? new Date() : null,
-		totalPurchases: isChecked ? increment(1) : increment(-1),
-	});
+	const { newEstimate, selectedLastPurchase } =
+		await handleCalculateEstimate(listRef);
+
+	//checks to see if checkbox is checked in front end, updates selected item
+	if (isChecked) {
+		await updateDoc(listRef, {
+			dateLastPurchased: [...selectedLastPurchase, new Date()],
+			dateNextPurchased: getFutureDate(newEstimate),
+			totalPurchases: increment(1),
+		});
+	} else {
+		selectedLastPurchase.pop();
+		await updateDoc(listRef, {
+			dateLastPurchased: [...selectedLastPurchase],
+			totalPurchases: increment(-1),
+		});
+	}
 }
 
 export async function deleteItem() {
diff --git a/src/utils/dates.js b/src/utils/dates.js
index dc66d95..ca68674 100644
--- a/src/utils/dates.js
+++ b/src/utils/dates.js
@@ -10,3 +10,15 @@ export const ONE_DAY_IN_MILLISECONDS = 86400000;
 export function getFutureDate(offset) {
 	return new Date(Date.now() + offset * ONE_DAY_IN_MILLISECONDS);
 }
+
+// This function will enable us to convert JS dates into Firebase-friendly timestamps, calculate the difference, and then return the difference between them (as a number).
+
+export function getDaysBetweenDates(firstDate, secondDate) {
+	// Takes two JS Date objects, converts to MS, calculates MS difference, then converts to days as a number (rounded up or down).
+	const firstTime = firstDate.getTime();
+	const secondTime = secondDate.getTime();
+	const secondsBetween = secondTime - firstTime;
+	const daysBetween = secondsBetween / ONE_DAY_IN_MILLISECONDS;
+	// Round number here
+	return daysBetween.toPrecision(3);
+}