Timesheet periods determine the length of user Timesheets. They should parallel either your organization’s pay periods or billing cycle so that timesheet data is available when needed for generating payroll or for invoicing clients. A System timesheet period is assigned to new users by default. An administrator can use different Timesheet Period for different employees, according to different Working Days.
This feature is under Admin Options > Timesheet Period View
Under Timesheet Period Types, there are 5 types of Period view from which 1 can be used in Working Day.
It does sound that Bi-weekly and Semi-monthly are similar but there is a difference.
In BiWeekly, UNItekTIME allows an administrator to change start period date according to the organizational need (as per below picture shows). Whereas, in Semi-monthly, periods are fixed from 1st to 15th and 16th to the end of the month date.
In Monthly Timesheet Period, UNItekTIME allows an administrator to set Month Start Date according to the organizational need.