Abstract
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 $\mathrm{SGR}1935+2154$, deriving a window period of 231 days and 55% duty cycle.