Unix time is a system for describing a point in time, defined as the number of seconds that have elapsed since January 1st, 1970.
You can convert dates to Unix time using an online calculator such as:
https://www.unixtimestamp.com/
Unix time is used in Telebroad's API methods to set the duration of a request with the "start" and "end" parameters.
For example if you want to have all SMS messages shown for the appropriate method use start=0 and end=-1, meaning the start time will be January 1st, 1970 (earliest possible time in Unix) and the end time will be one second ago from the present.