performance-dayjs-usage

title: Day.js Performance Guidelines
impact: HIGH
impactDescription: Significant performance improvement in date-heavy operations
tags: performance, dates, dayjs

Day.js Performance Guidelines

**Impact: HIGH (Significant performance improvement in date-heavy operations)**

Day.js with the `@calcom/dayjs` wrapper is heavy because it pre-loads all plugins including locale handling. Use alternatives when strict timezone awareness isn't required.

**Incorrect (using Day.js unnecessarily):**

// Slow in performance-critical code (loops)
dates.map((date) => dayjs(date).add(1, "day").format());

// Using Dayjs for simple date operations
const startOfMonth = dayjs().startOf("month");

**Correct (using performant alternatives):**

// Use .utc() for better performance when timezone doesn't matter
dates.map((date) => dayjs.utc(date).add(1, "day").format());

// Use native Date when possible
dates.map((date) => new Date(date.valueOf() + 24 * 60 * 60 * 1000));

// Use date-fns for simple operations
import { startOfMonth, endOfDay } from "date-fns";
const monthStart = startOfMonth(dateObj);
const dayEnd = endOfDay(dateObj);

// For browser locale, use Intl with i18n
const { i18n: { language } } = useLocale();
new Intl.DateTimeFormat(language).format(date);

**When to use Day.js:**

• When you need strict timezone awareness (e.g., in the Booker)
• When working with complex timezone conversions
• When the performance impact is negligible (non-loop operations)

**When to avoid Day.js:**

• Simple date arithmetic
• Date formatting without timezone concerns
• Performance-critical loops over dates

Reference: [Cal.diy Engineering Standards](https://cal.com/blog/engineering-in-2026-and-beyond)