  This work analyzes the variation over time of the effect of geographic
distance on knowledge flows. The flows are measured through the citations
exchanged between scientific publications, including and excluding
self-citations. To calculate geographic distances between citing and cited
publication, each publication is associated with a "prevailing" territory,
according to the authors' affiliations. We then apply a gravity model to
account for the research size of the territories, in terms of cognitive
proximity of citing-cited publications. The field of observation is the
2010-2017 world publications citing the 2010-2012 Italian publications, as
indexed in the Web of Science. The results show that in domestic knowledge
flows, geographic proximity remains an influential factor through time,
although with differences among disciplines and trends of attenuating effects.
Finally, we replicate the analyses of knowledge flows but with the exclusion of
self-citations: in this manner the effect of geographic proximity seems
reduced, particularly at the national scale, but the differences (with vs
without self-citations) lessen through time. As shown in previous works, the
effect of distance on continental flows is modest (imperceptible for
intercontinental flows), yet here too time has some influence, including
concerning exclusion of self-citations.
