1) you can't. I wouldn't worry about it.
2) Screen readers need <label>
s to be read correctly, you can do it like so
<label for="date">Birthday (DD/MM/YYYY)</label>
<input id="date" name="date" type="text"/>
If you want to hide the formatting, you can do:
<label for="date">Birthday <span class="hidden">DD/MM/YYY</span></label>
where hidden is:
.hidden {
position: absolute;
top: 0;
left: -999px;
}