Timezone Strategy

Always save timestamps in UTC using toISOString().

// GOOD
const record = {
  checked_in_at: new Date().toISOString()  // "2025-01-14T22:43:00Z"
}

// DB column type: TIMESTAMP WITH TIME ZONE

2. Frontend Display (JST)

Always specify timeZone: 'Asia/Tokyo' when displaying.

// GOOD
new Date(utcString).toLocaleTimeString('ja-JP', {
  hour: '2-digit',
  minute: '2-digit',
  timeZone: 'Asia/Tokyo'
})

3. Current Date/Time (JST)

Use utility functions from lib/utils/timezone.ts.

import { getCurrentDateJST, getCurrentTimeJST } from '@/lib/utils/timezone'

const today = getCurrentDateJST()  // "2025-01-15"
const now = getCurrentTimeJST()    // "07:43"

Best Practices

Do

• Use new Date().toISOString() for database storage
• Use timeZone: 'Asia/Tokyo' for display
• Use getCurrentDateJST() / getCurrentTimeJST() for current date/time
• Use TIMESTAMP WITH TIME ZONE column type

Don't

• Use new Date().toISOString().split('T')[0] for "today's date"
  • Returns UTC date, which is yesterday before 9:00 AM JST
• Omit timezone specification when displaying
• Store JST time directly in database

Utility Functions

Available in lib/utils/timezone.ts:

Notes

• Supabase Dashboard shows UTC (this is normal)
• JST 7:43 = UTC 22:43 (previous day) is correct conversion
• Japan does not use daylight saving time (fixed UTC+9)

References

• lib/utils/timezone.ts - Utility function implementations
• docs/03_database.md - Database schema definitions