Sometimes the credit you give has a lifespan -- that is, you only want to vouch for the achievement for a certain period of time. After that, perhaps the recipient needs to demonstrate their worthiness for the credit again. A good example of credits that might expire include: Credits representing professional credentials that are re-issued every two years after continuing education classes or exams are completed, or Credits representing awards given out annually declaring recipients reigning champions for the current year.
When you create or edit a Credit, you have the option to set its expiration period. The option always default to credits that never expire. If you want to designate a time period for expiration, click "Add more detail" while creating a Credit in the Credit Builder tool (under "Give" area of the site), and set the expiration accordingly in the "Expires" section. You can choose from days, months, or years and set the number accordingly.
The actual expiration date for a Credit is automatically set based at the time the Credit is issued to someone, and is based on the current date and time when the Credit is issued. The expiration date will then show up when anyone looks at a recipients earned Credit, letting them know if the Credit is still valid and active.