diff --git a/admin/index.php b/admin/index.php
index 6f87285dea7..30ba5cb2c43 100644
--- a/admin/index.php
+++ b/admin/index.php
@@ -315,6 +315,11 @@ if (!$cache and $version > $CFG->version) {  // upgrade
         $testsite = 'behat';
     }
 
+    if (isset($CFG->themerev)) {
+        // Store the themerev to restore after purging caches.
+        $themerev = $CFG->themerev;
+    }
+
     // We purge all of MUC's caches here.
     // Caches are disabled for upgrade by CACHE_DISABLE_ALL so we must set the first arg to true.
     // This ensures a real config object is loaded and the stores will be purged.
@@ -324,6 +329,11 @@ if (!$cache and $version > $CFG->version) {  // upgrade
     // We then purge the regular caches.
     purge_all_caches();
 
+    if (isset($themerev)) {
+        // Restore the themerev
+        set_config('themerev', $themerev);
+    }
+
     $output = $PAGE->get_renderer('core', 'admin');
 
     if (upgrade_stale_php_files_present()) {
diff --git a/lib/moodlelib.php b/lib/moodlelib.php
index 75b843fe23c..f1451d598c2 100644
--- a/lib/moodlelib.php
+++ b/lib/moodlelib.php
@@ -9620,35 +9620,6 @@ function get_course_display_name_for_list($course) {
     }
 }
 
-/**
- * Convert a Moodle Version formatted number into a guaranteed incrementing timestamp.
- *
- * @param float $version The version to convert
- * @return int The calculated timestamp
- */
-function version_to_timestamp($version) {
-    // Moodle version numbers are in stored in one of the following formats:
-    // * YYYYMMDDRR.II
-    // * YYYYMMDDRR
-    //
-    // To convert these into a value akin to a Unix Time Stamp, we take the timestamp of the YYYYMMDD component. and add
-    // a calculation of the RR.II component.
-
-    // The hours/minutes/seconds must be specified otherwise the current time is used.
-    $date = DateTime::createFromFormat('YmdHis', substr($version, 0, 8) . '000000');
-    $revision = $date->format('U');
-
-    // To ensure a unique incrementing timestamp, we multiple the $rr value by a value larger than the max of $ii,
-    // and then add $ii.
-    // As a two-digit number, the max value of $ii is 99, so we multiply by 100.
-    $revision += (100 * (int) substr($version, 8, 2));
-
-    // Note: If there is no .II value, or it is 00, then the substr will return false, but be calculated as 0 when cast to int.
-    $revision += (int) substr($version, 11, 2);
-
-    return $revision;
-}
-
 /**
  * The lang_string class
  *
diff --git a/lib/outputlib.php b/lib/outputlib.php
index b68e0707702..dc40d2bdc65 100644
--- a/lib/outputlib.php
+++ b/lib/outputlib.php
@@ -86,13 +86,8 @@ function theme_get_revision() {
 
     if (empty($CFG->themedesignermode)) {
         if (empty($CFG->themerev)) {
-            // If theme designer mode is not set, and there is no themerev, this is almost certainly part of the installation.
-            // We attempt to set a themerev based on the Moodle version number to avoid costly rebuilds of the dynamic
-            // theme files between each page load.
-            $version = null;
-            require("{$CFG->dirroot}/version.php");
-
-            return version_to_timestamp($version);
+            // This only happens during install. It doesn't matter what themerev we use as long as it's positive.
+            return 1;
         } else {
             return $CFG->themerev;
         }
diff --git a/lib/tests/moodlelib_test.php b/lib/tests/moodlelib_test.php
index bc9bf47ba73..af169a5de5e 100644
--- a/lib/tests/moodlelib_test.php
+++ b/lib/tests/moodlelib_test.php
@@ -3282,26 +3282,4 @@ class core_moodlelib_testcase extends advanced_testcase {
              'samecourse' => false, 'result' => false],
         ];
     }
-
-    /**
-     * Unit tests for the version_to_timestamp function.
-     */
-    public function test_version_to_timestamp() {
-        $baseversion = 2016110100.00;
-        $basestamp = version_to_timestamp($baseversion);
-        $this->assertEquals(1477929600, $basestamp);
-
-        // Adding 00.99 to the version should increase the timestamp.
-        $stamp = version_to_timestamp($baseversion + .99);
-        $this->assertGreaterThan($basestamp, $stamp);
-
-        // Adding 01.00 to the base version should increase the stamp higher than the 00.99 version.
-        $newstamp = version_to_timestamp($baseversion + 01.00);
-        $this->assertGreaterThan($basestamp, $newstamp);
-        $this->assertGreaterThan($stamp, $newstamp);
-
-        // The previous day's timestamp at it's highest increment should be lower than the base version.
-        $stamp = version_to_timestamp($baseversion - 00.01);
-        $this->assertLessThan($basestamp, $stamp);
-    }
 }