In a default OTRS installation 9 calendars are available, which can be configured graphically. Using the SysConfig framework found in the admin interface, this number can be increased to 99, by using the calendar 1 etc. links:
"TimeWorkingHours" can be used in OTRS::ITSM to define so-called "service level windows" time frames in which your organization guarantees certain service levels to your customers. If required, these can then be monitored and/or evaluated to ensure compliance with any possible Service Level Agreements.