Hey @Becky C - are you on Shopify or another ecommerce platform? If you’re on Shopify, according to Klaviyo’s doc (How to Create Unique Coupon Codes for Shopify - Create your Coupon Code in Klaviyo), this is what I found:
Please note that, in Klaviyo, dates are set in the UTC time zone; however, Shopify sets the expiration date/time based on the timezone the store is set to in Shopify, so you may see a different expiration dates in Klaviyo and Shopify for the same coupon.
Since Klaviyo generates the code from the platform itself, I think the expiration time is based on your timezone settings you set in your ecommerce platform, and expires until the date changes (in that timezone). In other words: 11:59 PM (and 59 seconds?) of the timezone you set in your ecommerce platform with all the usual caveats of Daylight Savings Time, Leap Year, taken into account.