This is all well and good:

No topics in the selected time period
Choose a longer time period to find some.

…except when there is no longer time period:

Probably the subtitle just shouldn't appear when the time period is set to "all time".

Alternatively, if the query isn't too expensive to run on the whole history, it could even change based on whether there actually were results found that ended up not being shown due to the selected time period.