Yes. If you are an administrator you can define any calendar you like to the system.
Go to Settings and select Calendars. select the + Add Calendar button at the top of the central panel and give the calendar a name. The calendar will appear in the list.
Next, you need to populate the calendar with date periods. For instance, if you are building a fortnightly calendar, then you need to define the time period for each fortnight and give then names. Click on the + Add Period button and define each period. This process has to be repeated for every calendar period you want to appear in your calendar.
Obviously, this easy for annual calendars and more tedious for shorter length period calendars. As we can’t guess what your custom calendar length or period length is, that’s about as much help we can provide!