What is best practice when dealing with date values without time, in JavaScript? My concern is subtle off-by-one bugs caused by local Time Zone (TZ) offset (e.g. +5 hours), when doing date math.

JavaScript's built-in Date type represents a date and time not just the date. Internal representation is an integer of microseconds since 1970-01-01 00:00:00 UTC. Other languages, like Python, have separate Date and DateTime types. Java 8 introduced LocalDate.

You also have things like: new Date('5/18/2020') is local TZ (in US), but new Date('2022-05-18') is UTC. Same with Date.parse(string). And the time zone on most servers in UTC, whereas on the browser side the time zone will vary.

The date values will be used for simple date math in code and will be stored in a SQL database.

Possibilities:

1. Use the an alternate type like integer value, of milliseconds or days (since 1970-01-01), or string in YYYYMMDD format.

2. This was combined with #1

3. Use Date ignoring time (as 00:00 local TZ). Convert from/to UTC when reading/writing to database

4. Use Date with time as 00:00 UTC. Have to be careful not to mix with Date values in local TZ (e.g. now = new Date())

5. Use Date in local TZ, but convert to UTC when read/writing to database. This is a variant of #3.

6. Create a LocalDate class that enforces midnight.

7. Use a library. js-joda has LocalDate.

I am leaning towards #3 and #6. Some code I am writing:

class LocalDate extends Date {
  // Error if time isn't midnight local TZ
  // Do not accept string in ISO format
  constructor(date?: Date|number|string)

  // Convert to 00:00 UTC
  // To be used before write to database
  toUtc(): Date

  // Only return date.  Also affects toJSON()
  toISOString(): string

  // Returns today's date at 00:00 Local TZ
  static today(): LocalDate
  // Set time to midnight local TZ, without error check.
  static fromDateTime(date: Date|number): LocalDate
  // Convert to local TZ.  Error if not 00:00 UTC.
  static fromUtc(date: Date|number): LocalDate
}

UPDATE:

Various edits.