diff --git a/services/web/migrations/20210726083523_convert_split_tests_assigned_at_strings_to_dates.js b/services/web/migrations/20210726083523_convert_split_tests_assigned_at_strings_to_dates.js
new file mode 100644
index 0000000000..4d34e1c204
--- /dev/null
+++ b/services/web/migrations/20210726083523_convert_split_tests_assigned_at_strings_to_dates.js
@@ -0,0 +1,11 @@
+const updateStringDates = require('../scripts/split_tests_assigned_at_to_dates')
+
+exports.tags = ['saas']
+
+exports.migrate = async client => {
+  await updateStringDates()
+}
+
+exports.rollback = async client => {
+  /* nothing to do */
+}
diff --git a/services/web/scripts/split_tests_assigned_at_to_dates.js b/services/web/scripts/split_tests_assigned_at_to_dates.js
new file mode 100644
index 0000000000..9df234cbb6
--- /dev/null
+++ b/services/web/scripts/split_tests_assigned_at_to_dates.js
@@ -0,0 +1,47 @@
+const { db, waitForDb } = require('../app/src/infrastructure/mongodb')
+
+async function updateStringDates() {
+  await waitForDb()
+  const users = await db.users.find({
+    splitTests: { $exists: true },
+  })
+
+  let user
+  let count = 0
+  while ((user = await users.next())) {
+    count += 1
+    if (count % 10000 === 0) {
+      console.log(`processed ${count} users...`)
+    }
+
+    const splitTests = user.splitTests
+    for (const splitTestKey of Object.keys(splitTests)) {
+      for (const variantIndex in splitTests[splitTestKey]) {
+        splitTests[splitTestKey][variantIndex].assignedAt = new Date(
+          splitTests[splitTestKey][variantIndex].assignedAt
+        )
+      }
+    }
+
+    await db.users.updateOne(
+      {
+        _id: user._id,
+      },
+      { $set: { splitTests } }
+    )
+  }
+  console.log(`Updated ${count} assignedAt strings to dates!`)
+}
+
+if (!module.parent) {
+  updateStringDates()
+    .then(() => {
+      process.exit(0)
+    })
+    .catch(error => {
+      console.error(error)
+      process.exit(1)
+    })
+}
+
+module.exports = updateStringDates