What is a unix timestamp?
The Unix epoch (or Unix time or POSIX time or Unix timestamp) is the number of seconds that have elapsed since January 1, 1970 (midnight UTC/GMT), not counting leap seconds (in ISO 8601: 1970-01-01T00:00:00Z). The current unix time can be found here: http://www.unixtimestamp.com
Here's how to convert a timestamp to be used in the ClickSend Zapier action: http://help.clicksend.com/zapier/schedule-a-message-with-zapier
Here's how to generate a UNIX timestamp using PHP: http://php.net/manual/en/function.strtotime.php