The thematic map in SWAT Output Viewer is able to show data of three summary types: single time step, annual and average annual. For single time step, it could be each day, each month and each year depending out the configuration of the outputs.
When the single time step option is selected, the time could be switched by double-clicking on the data record in chart table as shown below where monthly result is used. The time is also shown in the in the status bar on the bottom left corner. Note that, this only affect the thematic map. The chart would be affected.
Michael Yu 