  The DAMA/LIBRA collaboration has reported the observation of an annual
modulation in the event rate that has been attributed to dark matter
interactions over the last two decades. However, even though tremendous efforts
to detect similar dark matter interactions were pursued, no definitive evidence
has been observed to corroborate the DAMA/LIBRA signal. Many studies assuming
various dark matter models have attempted to reconcile DAMA/LIBRA's modulation
signals and null results from other experiments, however no clear conclusion
can be drawn. Apart from the dark matter hypothesis, several studies have
examined the possibility that the modulation is induced by variations in their
detector's environment or their specific analysis methods. In particular, a
recent study presents a possible cause of the annual modulation from an
analysis method adopted by the DAMA/LIBRA experiment in which the observed
annual modulation could be reproduced by a slowly varying time-dependent
background. Here, we study the COSINE-100 data using an analysis method similar
to the one adopted by the DAMA/LIBRA experiment and observe a significant
annual modulation, although the modulation phase is almost opposite to that of
the DAMA/LIBRA data. Assuming the same background composition for COSINE-100
and DAMA/LIBRA, simulated experiments for the DAMA/LIBRA without dark matter
signals also provide significant annual modulation with an amplitude similar to
DAMA/LIBRA with opposite phase. Even though this observation does not explain
the DAMA/LIBRA's results directly, this interesting phenomenon motivates deeper
studies of the time-dependent DAMA/LIBRA background data.
