With a daily budget, lower traffic days may mean that your ads receive fewer impressions and have some budget leftover. We don't 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'll 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'll apply your unused budget to remaining business days to take traffic fluctuations into account.

Timeframe is the new adwords budgeting option that allows the advertisers to adjust their daily budget based on their traffic to reach a set dollar budget as per the calendar month automatically.

Tune in to Google help document for more information.

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