What is the best message to display?

This can depend on your store.

The point of Delivery Timer is to give enough basic information to reduce friction to checkout. A timer shows your site is “Live and active”. You can only show 1 set of countdown and delivery dates on each product page. This reduces information overload with your actual checkout shipping options further guiding the customer.

If you have multiple options, you should lead with your Premium service which is usually the quickest. Again, once in checkout, your customers will select the service based on the price and required delivery speed.

The crucial element of your store is the cutoff time which forms the countdown. If you send out packages on a working day at say 5pm, you may have a store policy of 4pm cutoff. This means a customer has up to 3.59.59pm to place an order for same day processing. At 4.01pm, their package will most likely be in the next working days post.

If your store is more relaxed than that in terms of when you process orders, for example you may drop ship, then you should probably have midnight as your cutoff. 23.59pm 

The delivery date is optional. Most merchants implement it if makes sense for the store to do so. For example, if you primarily ship products within one country, this is a great example to make use of the delivery date.

If you offer 1st Class/Next Day Delivery then you should use this as the lead for your delivery date. 

The message should convey something that can apply to the majority store audience. For example:

“Order within the next {{ countdown }} and choose next day delivery to get it by {{ deliverydate }}. Further options available at checkout.”

‘Further Options’ could then be linked to a store policy on Shipping for example. To create a link, check out this article.

Another example could be:

“Want it by {{ deliverydate }}? Order within the next {{ countdown }}”

The stores cart and checkout process will handle the true detailed results based on what the customer selects.