Is there a specific tag in html so as to make a field behave as a date picker in a form?

no such facility is there in html.
You may user jquery-ui, datepicker, its easy to use

Yes, in html5 there is: <input type="date"> . But it's not really widely supported.

ok found using the jquery codes to show picker

@twiss: Its not html5 in my case, just a normal html form

There's also type=datetime, if that's what you want. Support of it: http://www.quirksmode.org/html5/inputs.html. Turns out only Chrome 10+ and Opera support it. IE10 will probably support it, but it will take a long time for people to use that.
Edit; browsers don't care about the doctype for this, but using a script is better indeed.

am going for the javascript one in the end, seems rather simple to implement