In your case I would state something like the following but you will need to feel comfortable with the wording and how positive you wish to leave it and whether or not you wish to provide more than the 30 days. If it doesn't adversely affect your future employer.

Dear Mr./Ms. Manager:

Please accept this letter as my formal notice of resignation from (company name). I know it is company policy to give a 30 day notice, so based on my calculations my last day would be effective on (April 29, 2010). I have enjoyed my employment here and appreciate all I have learned.

I hope that this thirty day notice is sufficient for you to find a replacement for me. If I can help to train my replacement or we need to extend it a few days more to tie up any loose ends in the next thirty days, please let me know.

Thank you very much for the opportunity to work here.

Sincerely,

(your signature)

Your Name

Your confusion is EXACTLY why there needs to be a date. You and your supervisor need to understand, and agree, what your last date will be.

I had a co-worker that gave a 2-week notice, and my inexperienced supervisor didn't clarify his last day. The supervisor expected the 2 weeks started the day after the notice was give, while my co-worker started the "clock" the day he gave notice. So 2 weeks later we had a problem when my co-worker wasn't there.

As for the date, I think it's usually just by the calendar - 14 calender days (for a 2-week notice) or 30 calendar days in your case.

If you feel comfortable that your employer will not cause you problems, you could mention it to him/her verbally and then decide on a date together. Then give the written notice later that day.