How in mysql get records that...

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were added TODAY,
all added since 7 days ago till now...
all added since 30 days ago till now...
all added since 365 days ago till now...

I have a DATETIME column
for example... today is 2001-09-06
and I want to get all records added today
added from 2001-08-31 (7 days ago) till today 2001-09-06..... etc...

HOW TO DO IT???

I'm new in databases