v2.8 of this plugin is now available.

this version made with the zen cart 2.1.0 file set. That said this should work for versions back to 1.5.8 - just be careful with the file merges.

Most importantly, this version uses a new calendar tool - jquery datetimepicker. the previous calendar tool relied on the mootools library to function, and unfortunately it's presence caused some disfunctionality. as it has not been updated for many years an update to a later mootools library was not possible as a solution. the older calendar did offer more functionality, for example the ability to enter a range of dates to blockout (now you'll need to list each one individually), but aside from that you should be able to run your zen cart reliably with this version.

Added Delivery Date to Account/Account History Info

And this works with one page checkout 2.5.4 - and probably any older version.