Not sure I understand the question of where or how to decode it. Either way on the Remington Society of America on the Manufactured Date link there is a diagram and a table that answers both.
DATE STAMP -- The function to show current date and time in Excel is =NOW().Now Function
You can remove date stamp from image using Inpaint http://www.theinpaint.com/inpaint-how-to-remove-date-stamp-from-photo.html
you should date stamp a letter because then others know when it came in and when in can be distributed
Simplest and fastest way is to purchase a rubber date stamp. You can stamp the date on the back of the card above the manufacturer logo.
post card with 1 cent stamp date of 1911
date stamp
Date Stamp
When date stamping incoming mail, place the date stamp in the upper right corner of the envelope or on the front side where it is easily visible. If the envelope has a window, ensure the stamp does not obscure any important information. For packages, apply the date stamp on the shipping label or the top side of the package. This ensures clear documentation of when the mail was received.
There isn't any stamp that is called a penny purple. You need to describe the stamp - include country and if you can date and what is on the stamp.
If you're looking for the financial value of this particular postage stamp, you'll have to consult a "Stamp Collector's Book." The year, type of stamp, date of stamp, date of post mark, and condition of the stamp will have to be compared to that described in the collector's book. Almost all book stores sell stamp collector's books.
1765
Without knowing the date on the stamp, a description and condition, it is impossible to value a stamp.