I originally reviewed this at 1-star because it was totally non-functional. I determined the problem was an older PHP version active on website. After updating PHP to version 7.1 calendar was functional. That was my mistake, as I over-looked the error message on screen.
Calendar is great IF there are no modifications to either service price, services offered, or time/date worker is available. If any object changes, the database automatically reverts to its original configuration and new data is replaced with older data. The only way to use the new data, is to delete the connection, for that worker (depending on change, you may need to delete all workers), delete the old information you want to update, create new data and new connection. This is a database managing nightmare and too time-consuming to be user friendly.
If I want to change a workers time or day of week, I should not have to delete the entire connection to start it from scratch. If I want to change the price for all workers, I should not have to delete all worker connections to rebuild database. It should automatically update to the new information, not revert to old data.
Without these database connectivity problems, I would have rated it at least 4-stars. Because these problems are severe enough, I am unable to effectively use this plug-in and rating it at 2-stars.