<p>What is a mathematical way of of saying 1 - 1 = 12 for a month calculation?  Adding is easy, 12 + 1 % 12 = 1, but subtraction introduces 0, stuffing things up.</p>

<p>My actual requirement is x = x + d, where x must always be between 1 and 12 before and after the summing, and d any unsigned integer.</p>

Assuming x and y are both in the range 1-12:
```
```
((x - y + 11) % 12) + 1
```
```
To break this down:
```
```
// Range = [0, 22]
x - y + 11

// Range = [0, 11]
(x - y + 11) % 12

// Range = [1, 12]
((x - y + 11) % 12) + 1
```
```