How the rotation math works
Every fixed rotation is just a repeating list. The tool counts the whole days between the cycle start date and the date you are asking about, adds your starting position, and takes the remainder when divided by the length of the pattern. Because the arithmetic wraps in both directions, dates before the cycle start work as well as dates after it. A 4 on, 4 off rotation is an eight-day list, so the shift on any day depends on the number of days since the anchor modulo eight, and the pattern lands on a different weekday each cycle rather than repeating weekly.
The easiest way to calibrate the tool is to pick a day you already know from your posted schedule, put that date in the cycle start box, then change the starting position until the answer matches. Once that anchor is right, every other date in the past or future is correct too.
The common patterns
Four on, four off is an eight-day cycle giving 3.5 twelve-hour shifts a week, or 42 hours, with a four-day break coming around every eight days. Three on, three off is the six-day version, averaging 3.5 shifts a week as well but rotating through weekends faster. The 2-2-3 pattern, often called Pitman or a DuPont variant, runs over 14 days: two on, two off, three on, two off, two on, three off. It gives every crew a full weekend off every other week, which is why it is popular in policing, dispatch centers, refineries and hospitals.
The days and nights rotation in the list works two day shifts, two night shifts, then four days off. Rotating forward, from days to evenings to nights, is easier on sleep than rotating backward, and most industrial hygiene guidance recommends at least 48 hours off after a run of nights. For anything not in the list, choose the custom option and type your own sequence, for example D,D,D,N,N,O,O,O for a three-day, two-night, three-off cycle.
Using the 30-day table
The table shows the next 30 days from the date you looked up, with the weekday beside each code, so you can see at a glance which weekends you are working, when the long break falls and how many consecutive nights you have coming. Overtime under the Fair Labor Standards Act is generally owed on hours over 40 in a fixed seven-day workweek, and 12-hour rotations often produce alternating 36 and 48 hour weeks, so check how your employer defines the workweek before assuming a schedule averages out.