--- a/toolkit/components/telemetry/Histograms.json
+++ b/toolkit/components/telemetry/Histograms.json
@@ -13714,16 +13714,26 @@
     "record_in_processes": ["main"],
     "alert_emails": ["jhofmann@mozilla.com"],
     "bug_numbers": [1286118],
     "expires_in_version": "64",
     "kind": "boolean",
     "keyed": true,
     "description": "Permission requests (showing a permission prompt) by whether they were requested from code handling a user input event."
   },
+  "PROFILE_DIRECTORY_FILE_AGE": {
+    "record_in_processes": ["main"],
+    "alert_emails": ["jrediger@mozilla.com", "telemetry-client-dev@mozilla.com"],
+    "bug_numbers": [1458574],
+    "expires_in_version": "65",
+    "high": 1500,
+    "n_buckets": 20,
+    "kind": "exponential",
+    "description": "Age of files scanned in profile directory in days when determining initial profile age."
+  },
   "HTMLEDITORS_WITH_RESIZERS": {
     "record_in_processes": ["content"],
     "alert_emails": ["mnakano@mozilla.com"],
     "bug_numbers": [1452538,1449564],
     "expires_in_version": "65",
     "kind": "boolean",
     "releaseChannelCollection": "opt-out",
     "description": "Number of HTML editors which has shown object resizers."

--- a/toolkit/components/telemetry/Scalars.yaml
+++ b/toolkit/components/telemetry/Scalars.yaml
@@ -1219,16 +1219,30 @@ telemetry:
     keyed: true
     notification_emails:
       - telemetry-client-dev@mozilla.com
       - chutten@mozilla.com
     release_channel_collection: opt-out
     record_in_processes:
       - 'all'
 
+  profile_directory_scans:
+    bug_numbers:
+      - 1458574
+    description: >
+      The number of times a profile directory scan was initiated in ProfileAge.jsm
+    expires: "65"
+    kind: uint
+    notification_emails:
+      - jrediger@mozilla.com
+      - telemetry-client-dev@mozilla.com
+    release_channel_collection: opt-out
+    record_in_processes:
+      - 'all'
+
 telemetry.discarded:
   accumulations:
     bug_numbers:
       - 1369041
     description: >
       Number of discarded accumulations to histograms in child processes
     expires: "never"
     kind: uint

--- a/toolkit/modules/ProfileAge.jsm
+++ b/toolkit/modules/ProfileAge.jsm
@@ -1,21 +1,33 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
 var EXPORTED_SYMBOLS = ["ProfileAge"];
 
+ChromeUtils.import("resource://gre/modules/Services.jsm");
+ChromeUtils.import("resource://gre/modules/TelemetryUtils.jsm");
 ChromeUtils.import("resource://gre/modules/osfile.jsm");
 ChromeUtils.import("resource://gre/modules/Log.jsm");
 ChromeUtils.import("resource://services-common/utils.js");
 
 /**
+ * Calculate how many days passed between two dates.
+ * @param {Object} aStartDate The starting date.
+ * @param {Object} aEndDate The ending date.
+ * @return {Integer} The number of days between the two dates.
+ */
+function getElapsedTimeInDays(aStartDate, aEndDate) {
+  return TelemetryUtils.millisecondsToDays(aEndDate - aStartDate);
+}
+
+/**
  * Profile access to times.json (eg, creation/reset time).
  * This is separate from the provider to simplify testing and enable extraction
  * to a shared location in the future.
  */
 var ProfileAge = function(profile, log) {
   this.profilePath = profile || OS.Constants.Path.profileDir;
   if (!this.profilePath) {
     throw new Error("No profile directory.");
@@ -118,39 +130,48 @@ this.ProfileAge.prototype = {
   },
 
   /**
    * Traverse the contents of the profile directory, finding the oldest file
    * and returning its creation timestamp.
    */
   getOldestProfileTimestamp() {
     let self = this;
-    let oldest = Date.now() + 1000;
+    let start = Date.now();
+    let oldest = start + 1000;
     let iterator = new OS.File.DirectoryIterator(this.profilePath);
     self._log.debug("Iterating over profile " + this.profilePath);
     if (!iterator) {
       throw new Error("Unable to fetch oldest profile entry: no profile iterator.");
     }
 
+    Services.telemetry.scalarAdd("telemetry.profile_directory_scans", 1);
+    let histogram = Services.telemetry.getHistogramById("PROFILE_DIRECTORY_FILE_AGE");
+
     function onEntry(entry) {
       function onStatSuccess(info) {
         // OS.File doesn't seem to be behaving. See Bug 827148.
         // Let's do the best we can. This whole function is defensive.
         let date = info.winBirthDate || info.macBirthDate;
         if (!date || !date.getTime()) {
           // OS.File will only return file creation times of any kind on Mac
           // and Windows, where birthTime is defined.
           // That means we're unable to function on Linux, so we use mtime
           // instead.
           self._log.debug("No birth date. Using mtime.");
           date = info.lastModificationDate;
         }
 
         if (date) {
           let timestamp = date.getTime();
+          // Get the age relative to now.
+          // We don't care about dates in the future.
+          let age_in_days = Math.max(0, getElapsedTimeInDays(timestamp, start));
+          histogram.add(age_in_days);
+
           self._log.debug("Using date: " + entry.path + " = " + date);
           if (timestamp < oldest) {
             oldest = timestamp;
           }
         }
       }
 
       function onStatFailure(e) {