Are there locales for dates?

Question

Not a programming question - but only applies to developers...

When saying "Do something tomorrow"

Is tomorrow always today's date + 1 and is that date the same throughout the world?
Only talking dates - no times.

Leap year? New year?

Answer 1

To answer the (old) question of "is [a] date the same throughout the world?", and ignoring the formatting of a date (DDMMYYYY vs. MMDDYYYY vs. YYYYMMDD, etc), my answer comes in two parts. For no apparent reason I've written it as a narrative:

1. Dates due to different time zones

As of this moment in time, my phone is saying it's the fifth day of the fourth month of the year 2015. I am in England, which is currently observing British Summer Time. As I am writing this, it is 01:35 BST. In New York, which is currently observing Eastern Daylight Time, my (fictional) identical twin is writing the same answer as me, but referring to 20:35 EDT of the previous day (i.e. the fourth day of the fourth month).

But then, the question said "Only talking dates - no times."...

2. Dates due to different calendars

I might travel to Thailand after writing this answer. Say I want the hotel I'm staying at to have a bowl of red M&Ms ready in my room for when I get there (as is my wont). I ask my (fictional) PA to email the hotel to put in this request.

By the time I get to the hotel, it'll be the sixth day of the fourth month (Indochina time, which Thailand is currently observing). My PA puts my arrival date as 06/04/2015 in the email (Thailand still uses the DDMMYYYY format despite adopting ISO 8601).

The official calendar of Thailand, however, is the Thai solar calendar, which is 543 years ahead of the Gregorian calendar which we in the Western world are used to. In the Thai solar calendar, I'll be arriving in the year 2558 (=2015 + 543). This isn't going to end well.

The receptionist at the hotel who receives the email has lived in Thailand all of their life and isn't aware of other calendars than the Thai solar calendar. Consequently, a request about red M&Ms for a customer who was due to arrive 543 years ago is ludicrous. It'd be as ludicrous as us seeing a use-by date on a can of spam of 1476 (=2015 - 543). The request is ignored, and I arrive at the hotel and am forced to pick the red M&Ms like some kind of animal.

All this because it was assumed that dates are the same around the world...

Answer 2

Consider this real world example of Western-Samoa in 2011. They switched their timezone that way that they crossed the international dateline border from east to west. The consequence was: The day 2011-12-29 was followed by 2011-12-31, leaving out the date 2011-12-30.

So we have an example that the term "tomorrow" in the astronomical sense of next solar day does not necessarily mean the next calendar date. And we can further see that dates are timezone-dependent and therefore not the same around our globe. Instead we have two lines on the globe where the date changes, namely the mignight line AND the international dateline border. It is therefore daily reality that we have different calendar dates around the globe (for example Australia in the morning is one day later in calendar than Europe in the evening at the same time).

About your remark regarding leap years or new year, that is another subject unrelated to the question if and when a calendar date changes locally or globally.