performance-dayjs-usage.md

title	impact	impactDescription	tags
Day.js Performance Guidelines	HIGH	Significant performance improvement in date-heavy operations	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.com Engineering Standards