Delete all emails before certain date for Gmail

To do this , you need to use some keyword when you search your email.

Example, let say i want to delete emails that contain sitemeter keywords that are older than 31, December 2011, i just enter "sitemeter before:2011/12/31" as shown below.

After that, click the box shown by the arrow to select all the emails and you will see one line of words just above your searched emails that let you select all the emails.

Once you select all the emails, just click the trash bin icon.

A confirmation box will pop up and just select "OK" and your emails will be deleted.

This example use just sitemeter as keyword and this might sometimes include emails that you don't intend to include. A better way is probably to use "from:reports@sitemeter.com before:2011/12/31" so only emails from sitemeter will get deleted. For more options, please refer to this link.