When to use "the" before a date

Question

When to use "the" before a date?

I came across the following sentence:

It dates back to the 6th century BC.

When I looked up when to use the before dates, I read an example sentence from Cambridge Grammar online resources. It says it is correct to write :

Today is the 7th September

Avoidably I am confused when to use the with dates and what does it mean? It is about style?

PS : I know there is already a bit confusion or maybe we can call it varieties of writing dates in terms of format because of regional differences and how to write a date was asked before so I will assume the following formats are standard ones. However, if you think the use of the is part of varieties of writing dates, you can include this point with giving different formats.

American English : September 7, 2016

British English : 7 September 2016

---

There is another thread asked regarding this issue on ELL but the answer is not enough , when we compare it with Cambridge Grammar online resources

There is another thread on ELU on general rules of writing formats.

Answer 1

You use "the" when referring to a time period rather than a date

In the 1700s

In the 6th century

You should also use "the" in certain date formats.

Today is the 7th of September

without "of", the above does not work.

Today is September 7th

is also acceptable.

I don't think I've ever heard someone from the UK say "today is 7th september", despite the date format being dd/mm/yyyy.

Answer 2

I've been searching information ona webpgae about the uses of time prepositions and I came across this photo. Three of them don't have the "of" and one of them has it. But I don't know know if the four are wrong or only those which don't have the "of".

Answer 3

whatever the writer pointed out to a certain date, they usually use 'the', and they also use it to mention a whole period of time such as the 80s or 90s and so on.