Welcome to Klaviyo Community @Serranopepper
Thank you for your great question!
The activation time for the Coupon Code is at the time of first send, not subsequent reminders.
Here is just a few tips -
Tip 1: Remember that coupon's expiration will always be 24 hours beyond what you set in the expiration date field, this is to allow for any delay that may happen during delivery.
Tip 2: In Klaviyo, dates are set in the UTC time zone. However, Shopify sets the expiration date and time based on the Shopify store’s timezone, so you may see different expiration dates in Klaviyo and Shopify for the same coupon.
Here is a useful link about Coupons for Shopify
https://help.klaviyo.com/hc/en-us/articles/115006155388-How-to-create-unique-coupon-codes-for-Shopify
Please let me know if you have any further questions and I will be happy to help.
Thanks,
Kylie