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.

Thanks so much for your help and detailed explanation, Joseph! We are on Shopify Plus so this gives me exactly what I needed. Have a fantastic day and thanks again!