Why I Standardize Every Node.js Project on UTC Time

// Runs at midnight local server time
const startOfToday = new Date();
startOfToday.setHours(0, 0, 0, 0);

const transactions = await db.collection("transactions").find({
  createdAt: { $gte: startOfToday }
}).toArray();

// Enforce UTC timezone at the absolute entry point
process.env.TZ = "UTC";

import express from "express";
import cron from "node-cron";
import mongoose from "mongoose";

const app = express();

// Simple logging middleware using UTC ISO 8601 strings
app.use((req, res, next) => {
  const timestamp = new Date().toISOString(); // Always outputs in UTC
  console.log(`[${timestamp}] ${req.method} ${req.url}`);
  next();
});

// A cron job that runs daily at midnight UTC
cron.schedule("0 0 * * *", () => {
  console.log(`[${new Date().toISOString()}] Running daily summary...`);
  generateDailyReport();
});

async function generateDailyReport() {
  const now = new Date();
  
  // Calculate yesterday's UTC boundaries
  const startOfYesterday = new Date(Date.UTC(now.getUTCFullYear(), now.getUTCMonth(), now.getUTCDate() - 1, 0, 0, 0, 0));
  const endOfYesterday = new Date(Date.UTC(now.getUTCFullYear(), now.getUTCMonth(), now.getUTCDate() - 1, 23, 59, 59, 999));

  console.log(`Aggregating records from ${startOfYesterday.toISOString()} to ${endOfYesterday.toISOString()}`);
  
  // MongoDB query using UTC ISO boundaries
  const transactions = await mongoose.connection.db.collection("transactions").find({
    createdAt: {
      $gte: startOfYesterday,
      $lte: endOfYesterday
    }
  }).toArray();

  console.log(`Processed ${transactions.length} transactions.`);
}

// Client-side JavaScript
const utcDateStr = "2026-05-31T09:30:00Z"; // From server API
const localDate = new Date(utcDateStr);

console.log(localDate.toLocaleString()); 
// Automatically displays in the user's browser timezone