When should Maryland Businesses fill out "Form 1 The annual report & business property return"?

When should Maryland Businesses fill out "Form 1 The annual report & business property return"?

In particular, does one fill it out for years prior or during the April 15th date of the current year?

If the form is actually an annual report, I’m confused how one fills it out for a year when it’s due on April 15th. The form suggests you have to start your year on April 15th and end it on April 15th, but companies have the right to choose when their fiscal year starts.