Project Billing Rules

Hi

Is there a way to set a start and end date for a time based billing rule?

The background is as follows:

• The billing rule is charge based
• Charges are created from time entered
• The time entered is in relation to one individual, who had a pay increase mid month. Accordingly, we want to revise the amount that is charged to the client via the project
• Time entered between the 1-15th of March will be charged to the client at $300 while 16-31st March will be at $350 an hour

I cannot see a start or end date in the project billing rules form or any place where I can specify the resource’s charge rate

thanks