Date Format

People in United States use date format of mm/dd/yyyy. British, Australia and New Zealand use dd/mm/yyyy. Format of yyyy-mm-dd is an ISO 8601 international standard, and is favored by European countries and all Chinese spoken countries. Canada is THE country currently using all of these three formats. Format of yyyy-mm-dd is official format for Canada. Because they are a Commonwealth member country, they also use British format. And because they are heavily influenced by United States, in day-to-day life Canadians use American format most.

It is for sure IT industry should use yyyy-mm-dd format, not because it is an ISO 8601 standard, but also it is only meaningful format when you conduct textual sorting. For instance, when you use date as part of file name, it can be directly sorted by Windows Explorer.

In Microsoft Excel, if yyyy-mm-dd format is needed, please go to Format Cells, chose Locale as Chinese (Hong Kong S.A.R). It can be found in Type.

Sunday is traditionally regarded as the first day of the week. Different industries may have their own standard. However, the International Organization for Standardization (ISO) specifies that the week begins with Monday, which is also the standard for IT industry. So, day 1 in a week is Monday. The week ends at 23:59:59 of Sunday. Beware, Microsoft Excel's function weekday() does use Sunday as day 1 in a week. In addition, in Microsoft SQL Server DATEPART(WEEKDAY, dateToCheck) does also generate 2 for Monday.

Look at following three times:

Time 1: 2000-01-01 11:59:59 AM
Time 2: 2000-01-01 12:00:00
Time 3: 2000-01-01 12:00:01 PM

How many seconds between Time 1 and Time 2 above? How many seconds between Time 2 and Time 3? Are you sure? Would you think again, please? We human being just would like to confuse ourselves. Then look at these:

Time a: 2000-01-01 11:59:59 PM
Time b: 2000-01-01 0:00:00
Time c: 2000-01-01 0:00:01 AM

Time d: 2000-01-01 0:59:59 AM
Time e: 2000-01-01 1:00:00 PM
Time e: 2000-01-01 1:00:01 AM

So, if it is possible, please use military time (24 hour time system) instead of 12 hour time system.

Look at following three times:

Time 4: 2000-01-01 00:00:00
Time 5: 2000-01-01 24:00:00
Time 6: 2000-01-02 00:00:00

Are they at same time? Indeed, the format of Time 5 would never exist in any official document, except in some railway time tables, where 24:00:00 used only to describe the arrival time. Time 4 and Time 6 are often confused, though there is 24 hours difference. However, for the purpose of reporting period ending time, it is suggested to use format either as 1999-12-31 24:00:00 or 1999-12-31 23:59:59, but not 2000-01-01 00:00:00. This is particularly important for week report, because the reader could not tell from the glance at the date. For instance, 2010-04-12 00:00:00, how can you directly tell it is one second from 2010-04-11 00:00:01 or 2010-04-12 23:59:59? No, both are incorrect. The correct answer is it is one second from 2010-04-12 00:00:01 or 2010-04-11 23:59:59.

When the first week starts in a year? ISO 8601 provides the first week in a year starts from a Monday of a week with includes the first Thursday in that year. Therefore, the first week must include the January 4th. This approach does indeed emphasis the majority of the days in first week are in new year. This approach can also be used to determine the first week of a fiscal year as well as the first week of a month.

http://en.wikipedia.org/wiki/Date_and_time_notation_by_country
http://en.wikipedia.org/wiki/Calendar_date
http://en.wikipedia.org/wiki/ISO_8601
http://24hourtime.info/the-24-hour-time-system/
http://www.cl.cam.ac.uk/~mgk25/iso-time.html
http://en.wikipedia.org/wiki/12-hour_clock