RFC DocumentsRFC 6238 - TOTP: Time-Based One-Time Password Algorithm4.1. NotationsOn this page4.1. Notations4.1. Notations X represents the time step in seconds (default value X = 30 seconds) and is a system parameter. T0 is the Unix time to start counting time steps (default value is 0, i.e., the Unix epoch) and is also a system parameter.