Hi Jorge,
I read somewhere in Bruce's posts that, when you define a lookup and the lookup field is a date field, the date picker will appear.
The "somewhere" would be above the browse, below the browse or both. According to your locator settings in the browse definition. Also, the date field to be filtered must be in the browse.
When the date field is not in the broiwse you might need to define the browse in a form an create a local date field in the form. On change you can filter the browse.
NTWS 5 is needed though.
Rene