This is only possible when sending messages from P3console via P3fax.
When sending a message through P3console you can select a date and time you wish your message to be sent by clicking on the Options button on P3console's Send Message dialog before sending.
Here you can select:
You can also control this function using P3fax embedded commands. For more information on P3fax embedded commands, select Start, Programs, P3fax, Help, Embedded commands - this document requires a PDF viewer, such as Adobe Reader, to be installed on your computer.