  Two repeating FRB sources, FRB180916 (CHIME/FRB Collaboration, 2020a) and
FRB121102 (Rajwade, et al., 2020, Cruces, et al., 2021) display periodic
windowed behavior (PWB) in the times of FRB detections. In PWB, events only
occur within a periodic window spanning some fraction of the period, but no
events may occur during many of the periods. During UT 2020 April 28, two peaks
of a soft gamma burst (Li, et al., 2021) were identified with two FRB peaks
(CHIME/FRB Collaboration, et al., 2020b) from soft gamma repeater (SGR)
1935+2154. As SGRs are a type of magnetar, these observations establish a link
between at least some FRBs and magnetars. The analysis herein, which selects
the period giving the minimum window fraction, shows PWB in this source's soft
gamma-ray emission. For 161 bursts from 2014 through 2020 from IPN
(Interplanetary Network) instruments, a clearly resolved minimum in window
fraction vs. period occurs at a 231 +/- 9 day period, and 55 \% window
fraction. The data cover only 6 bursting episodes, however, the periodicity
result appears relatively robust: First, the IPN has excellent time coverage
and is unlikely to miss bursts outside of these windows. Second, for various
data subsets (with significantly less bursts), either the best or second best
measured period was consistent with 231 days, even in the sub-sample with the
richest bursting episode removed. In addition, simulations show that even small
numbers of uniform random bursts do not show PWB, and that the results are not
due to limited sampling. The apparent PWB could have alternative explanations;
a firm conclusion of PWB would be evident, however, after the observation of
more consistent episodes. If the periodicity of this SGR's bursts can be
verified, and if similar behavior were observed in other repeating FRB sources,
it might suggest a linkage between FRB and SGR burst mechanisms or emission
conditions.
