style:date-style might require multiple number:year

We currently have

15.14.5 <number:date-style>
The <number:date-style> element specifies a style for date values.
This element can contain one instance of each of the following elements: ...

There is a problem with the "can contain one instance". There are lots of use cases where a user may want to have the year or month appear in multiple copies, for example to specify the year or month using two different calendars:
Tagu (January)
2136 (2009)
or even with the same calendar in two versions:
30 May (5) 2009