use monotonic time for updates

index.js

@@ -6,6 +6,15 @@ var express = require('express');
 
 const POLLING_TIMEOUT = 60000/*ms*/;
 
+let nextMonotonicTime = 1;
+
+function monotonicTime() {
+  const now = +new Date();
+  const mono = (now >= nextMonotonicTime) ? now : nextMonotonicTime;
+  nextMonotonicTime = mono + 1;
+  return mono;
+}
+
 var appendJournal = (function() {
   let lastJournalText = null;
   return function appendJournal(text) {
@@ -53,6 +62,15 @@ const dbInit = (async function dbInit() {
     CREATE INDEX IF NOT EXISTS games_modified ON games(modified)
   `);
 
+  const maxModified = await db.getAsync(`
+    SELECT MAX(modified) as result FROM games
+  `).catch(logDbError('maxModified'));
+
+  /* Just in case the system clock moved backward since the last record was written. */
+  if (maxModified && maxModified.result) {
+    nextMonotonicTime = maxModified.result + 1;
+  }
+
   console.log('Connected to the SQLite database.');
 
   return db;
@@ -286,7 +304,6 @@ function validateUpdate(body) {
 async function postGameHandler(req, res, next) {
   res.set('Cache-Control', 'no-store');
 
-  const time = +new Date();
   const gameId = req.params.gameId;
   const body = validateUpdate(req.body);
 
@@ -300,6 +317,8 @@ async function postGameHandler(req, res, next) {
     return;
   }
 
+  const time = monotonicTime();
+
   const params = {
     $gameId: gameId,
     $lightName: '',