To understand the dates on these big Sudanese coins made out of billon (an alloy of silver and copper, mostly copper), you first need to understand Eastern Arabic numerals. Second, you need to understand that these are Hijri dates based on the life of Mohammed and a lunar year. The AH1312 (Hijri) date is equivalent to Western dates around 1895A (Gregorian).
These coins are quite rare and very interesting. Coin collecting has not caught up with them yet, and they are still relatively inexpensive. In CoinQuest's opinion, this will change rapidly and their price will rise as more and more people are exposed (via the Internet) to these fascinating collectibles.
The catalogs quote the following values. All dates are roughly the same in value, except those coins with AH1311, which catalog about three times higher.
worn: $12 US dollars approximate catalog value
average circulated: $50
well preserved: $100
If you have one of these in good condition, congrats. But watch out for counterfeits. There is a good discussion of a fake 20 piastres at this Coin Community thread.