Quote:
So if you buy an issue on the last day offered before the new issue, then your usage of the issue in the days/weeks to come would be out of period?
It would be out-of-period for me. But there out-of-period usages which I find acceptable.
The following cover was mailed during the 36 cent period with a 34 cent stamp. The 34 cent stamp is out-of-period but affixed shortly after the rate change and the release of the 36 cent stamp.

I would love to have a cover mailed after March 13, 2014 franked with a 63 cent stamp and taxed. A cover with a 63 cent stamp and uprated to 85 cents (63c + 22c) would be interesting but not something that would appeal to me.
Commemoratives are a bit different. Generally I don't mind if a commemorative is used in the year it was issued. The 1970 Expo stamp was issued on March 18. This July cover is acceptable:
The Permanent Stamp Era : 2006 - 2013The first "P" stamp was issued on November 16, 2006 during the 51 cent letter rate period.

Although the letter rate increased to 52 cents on January 16, 2007, the 52 cent permanent stamps were issued on December 27, 2007.

I consider that the first "P" stamp was in-period from November 16, 2006 to December 26, 2007, i.e. until it was replaced by the "P" definitive stamp issued on December 27, 2007.
That really sounds confusing, doesn't it?
QUIZ TIME1. Why is this an in-period usage of the 5 cent Karsh definitive? The 5 cent Wilding definitive was issued on April 1, 1954!

2. Is this in-period or out-of-period?

It is a good idea to know when stamps were issued when buying covers. Out-of-period usages can be very disappointing. Dealers are not likely to advise customers, so it is essential to be vigilant.