PHP if statements required for WP shopping cart plugin

I have the wp-ecommerce plugin installed and require some simple php 'if' statements implementing server time. The site sells products that are only available at certain times of the day and that are also unavailable on certain days. The plugin has a feature called 'fancy notifications'. When a user clicks the add to cart button, a small ajax generated window appears notifying the user of what they have added. I require this notification to display a message between certain times of day. IE: when the products are unavailable. Also I require the window to display a message on a certain day - all day. For example: Sunday would be 'sorry we are closed all day today'. I do not require you to write the message, I can do that bit. I also need the script to be editable so I can change times and days as required. The script should also be implemented in the header of the site where the shopping cart contents are displayed, so the cart appears accordingly to time of day specified.