There are a wide variety of calendars, date and time systems, and systems of measurement.
The so called Christian calendar is the de facto standard for business correspondence around the world. Note that in many countries, regions and religiously divided areas there are other calendars in use there. Just a few of the countries with different calendars include China, Israel, Japan, Thailand and the Moslem world.
To add to the confusion, Americans and Europeans write the dates differently:
For example, one of the most important dates in human history is written:
- 7/20/69 by Americans and
- 20/7/69 by Europeans.
While in this case - with a 'date' bigger than 12 - we may guess, but in the case of a date like 7/4, who would be able to read the writer's mind? To make matters even worse, some use commas, periods, hyphens to separate the numbers. To be on the safe side, it would be better to write out the name of the month, and prevent confusion.
Also, if you are sending an email or similar communication and want to establish a time, you should indicate the time zone or use Greenwich Mean Time. Don't forget, some people use the 24 hour clock!
Then, to continue there is the matter of weights and measures. It may be useful to visit
this outside link for more info on those. Note that this link is from a British perspective, and spelling, numerical punctuation and related terms may be slightly different than the American perspective.