According to AdWordsAdvisor, the official Google mouthpiece at the WebmasterWorld, we now have confirmed reports that Google is carrying out a beta test on a new AdWords budgeting option is named “Timeframe” and offers advertisers the capability to shape their budget by month or by day.
The snippet below gives a better picture of the difference between monthly and daily budget option as:
With a daily budget, lower traffic days may mean that your ads receive fewer impressions and have some budget leftover. We do not use this leftover budget, but we do attempt to compensate for the loss in traffic by serving impressions up to 20% over the daily budget on high traffic days. However, some high traffic days require budget flexibility beyond this amount.
With a monthly budget, we will adjust your budget automatically each day to meet traffic demand while still respecting your budget for the calendar month. For example, if your ads often receive less traffic on non-business days during a month, we will apply your unused budget to remaining business days to take traffic fluctuations into account.
For more information on how this works, read the Google help document.