rdisp/startup_time is a SAP Parameter attribute which is used to control Delay before starting the first schedule information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
This parameter defines the time period after which the periodic tasks
(such as, the auto-ABAP) are to be executed after the startup.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.