Should it be "Apr. 15 to May 14" instead?

There’s nothing wrong with using a hyphen, it’s common practice.

However, it sounds American as they use the month day format, in the UK it’s more common to say, 15th April to 14th May

Date and time notation in the United Kingdom records the date using the day–month–year format (22 December 2020 or 22/12/20).

https://en.wikipedia.org/wiki/Date_and_time_notation_in_the_United_Kingdom#:~:text=Date%20and%20time%20notation%20in%20the%20United%20Kingdom%20records%20the,(1%3A27%20pm).