fix: timezone inconsistency in date type parsing

[simplenotezy]

Problem

The date type handler has inconsistent timezone handling between serialization and parsing, causing incorrect date values when the system timezone differs from UTC.

Root Cause:

• Serialization: toISOString() always outputs UTC format
• Parsing: new Date(x) interprets strings without timezone info as local time

This asymmetry creates a double timezone conversion bug affecting PostgreSQL operations like AT TIME ZONE, DATE_TRUNC(), and any timestamp operations in non-UTC system timezones.

Example Bug:

// System timezone: Europe/Copenhagen (UTC+2)
// Database timezone: UTC

const result = await sql`
  SELECT '2025-04-01 00:00:00'::timestamp AT TIME ZONE 'UTC' as timestamp_utc
`

// Expected: 2025-04-01T00:00:00.000Z
// Actual:   2025-03-31T22:00:00.000Z  (2-hour shift)

Solution

Modified the parse function in the date type handler to treat strings without timezone information as UTC, matching the serialization behavior:

parse: x => {
  if (typeof x === 'number') {
    return new Date(x)
  }
  // Fix timezone bug: ensure strings without timezone info are treated as UTC
  // to match serialization behavior (toISOString() always outputs UTC)
  if (
    typeof x === 'string' &&
    !x.includes('Z') &&
    !x.includes('+') &&
    !x.includes('-', 10)  // Avoid matching date part
  ) {
    // PostgreSQL format: "2025-04-01 00:00:00" -> ISO format: "2025-04-01T00:00:00Z"
    return new Date(x.replace(' ', 'T') + 'Z')
  }
  return new Date(x)
}

This ensures:

• timestamp columns are parsed as UTC to match serialization
• timestamptz columns with timezone info remain unchanged
• Round-trip consistency across different system timezones
• Backward compatibility for properly formatted timestamps

Testing

Added comprehensive test cases covering:

• Round-trip serialization/parsing consistency
• PostgreSQL timestamp format parsing
• AT TIME ZONE operation correctness

Related Issues

Fixes:

• Timestamp without timezone to Date conversion is incorrect #1075 - Timestamp without timezone to Date conversion is incorrect
• Library overriding timestamp with the wrong timezone #999 - Library overriding timestamp with the wrong timezone
• Select timestamp column not returning UTC #563 - Select timestamp column not returning UTC
• Timezone Issues #388 - Timezone Issues
• Timestamps are inconsistent with Postgres behavior electric-sql/pglite#532

Impact: Resolves timezone inconsistencies for applications running in non-UTC timezones and ensures consistent behavior across different system locales.