Being clear about any restrictions on the format of data that a user must enter will help them to avoid any errors. For example, specifying that date fields should be entered as DD/MM/YY will avoid them attempting to enter data in an incorrect format.
Provide clear examples that demonstrate what format any input should take. For example, indicate whether dates should be specified as day/month/year (UK) or month/day/year (US).
// Demonstrating the format of data in the label
<label for="date">Date (dd-mm-yy)</label>
<input type="text" name="date" id="date" />