Hi 'dude,
(Constructed in Numbers, but behaviour is essentially the same in Pages.)
Date formats apply only to the date part of date and time values. Date and Time values specify a single instant in time, and can be formatted to show only the date part,

All of the filled cells in this table contain the formula TODAY(), which returns a Date and Time value for today's date at the time 00:00:00 (midnight, at the beginning of the day.)
All but the last are formatted to show only the Date part, with the elements arranged in the order shown, and formatted as shown.
The bottom cell contains the same value, to which a Custom format has been applied. The format shows only the monthname (short form) and the year (including the century digits).
To show a range of dates (as you wish to do), you'll need to enter the value as TEXT. the easiest way to do that is to start with an apostrophe, which signals numbers that what follows is text, not a number or a date. Numbers then displays the entry as it was entered, omitting the ' at the beginning.
Same table, with a row added to accomodate entry of your example, entered as described here:

Formatting on the cell was automatically changed from Custom to Text by Numbers, as evidenced by the default text alignment to the left edge of the cell. (This behaviour also applies in Pages tables.)
As the cell's data is text, it can't be used as a date in formulas.
Regards,
Barry