Date & Timezone Handling

The Problem

HTML5 date inputs return strings like "2024-01-15". When parsed with new Date("2024-01-15"), JavaScript treats this as UTC midnight, not local midnight. This causes dates to display incorrectly across timezones.

Example bug: User in Pacific timezone (-8) selects Jan 15 → stored as Jan 15 00:00 UTC → displayed using getDate() → shows Jan 14 (because UTC midnight is 4pm previous day in Pacific).

Solution: Use UTC Consistently for Date-Only Fields

For fields that represent calendar dates (not specific moments in time):

1. Parse form strings as UTC midnight
2. Display using UTC methods

Utilities in src/utils/date-utils.ts

For Form Inputs

import { parseDateInputAsUTC, formatDateInputFromUTC } from "@/utils/date-utils"

// Parsing: form string → Date for server
parseDateInputAsUTC("2024-01-15") // → Date at UTC midnight

// Formatting: Date from DB → form string
formatDateInputFromUTC(date) // → "2024-01-15"

For Display

import { formatUTCDateShort, formatUTCDateFull, formatUTCDateRange } from "@/utils/date-utils"

formatUTCDateShort(date)  // → "Jan 15"
formatUTCDateFull(date)   // → "Jan 15, 2024"
formatUTCDateRange(start, end) // → "January 15-17, 2024"

When to Use What

Scenario	Use
Date-only form field (start date, end date)	parseDateInputAsUTC on submit, formatDateInputFromUTC for default value
Displaying calendar dates	formatUTCDateShort, formatUTCDateFull, formatUTCDateRange
Timestamps (createdAt, updatedAt)	Local time methods are fine (these are actual moments)

Common Mistakes to Avoid

// BAD - parses as UTC, displays as local
new Date("2024-01-15")
date.getDate()
date.toLocaleDateString()

// GOOD - consistent UTC handling
parseDateInputAsUTC("2024-01-15")
date.getUTCDate()
formatUTCDateFull(date)

Form Pattern

// In form component
import { parseDateInputAsUTC, formatDateInputFromUTC } from "@/utils/date-utils"

// Default values (for edit forms)
const form = useForm({
  defaultValues: {
    startDate: formatDateInputFromUTC(existingData.startDate),
  }
})

// On submit
function onSubmit(data) {
  saveToServer({
    startDate: parseDateInputAsUTC(data.startDate),
  })
}