If you only wish to control how long each voucher shows on your site, you can create a secondary extra field of date type (call it e.g. "expiration time") and use it to control when the voucher (extra field) stops displaying up on the site.

Use this way getk2.org/documentation/tips-a-tricks/display-single-extra-fields-anywhere-in-your-k2-content to retrieve both these 2 extra fields separately from the main extra fields loop in the item.php override (and elsewhere).

Then use the value of the "expiration time" extra field to show or hide the voucher extra field value.