  Many astrophysical bursts can recur, and their time series structure or
pattern could be closely tied to the emission and system physics. While
analysis of periodic events is well established, some sources, e.g. some fast
radio bursts and soft gamma-ray emitters, are suspected of more subtle and less
explored periodic windowed behavior: the bursts themselves are not periodic,
but the activity only occurs during periodic windows. We focus here on
distinguishing periodic windowed behavior from merely clustered events through
time clustering analysis, using techniques analogous to spatial clustering,
demonstrating methods for identifying and characterizing the behavior. An
important aspect is accounting for the ``curious incident of the dog in the
night time'' - lack of bursts carries information. As a worked example, we
analyze six years of data from the soft gamma repeater SGR1935+2154, deriving a
window period of 231 days and 55% duty cycle; this has now successfully
predicted both active and inactive periods.
