"The substrate can be tested but how can one know when the footprints were actually made." - Fisherman
The substrate would need to be soft enough to allow indents to be made, therefore lithification had not occurred at the point the prints were made.
The prints must have been made during a period of deposition because if erosion was occurring we could not still see them as they would have been eroded at the time of making. Therefore as more layers were placed on top, compaction would begin the lithification process.
After the lithified strata was eroded away revealing this particular layer, the now visible surface would be too hard to leave these impressions.
Therefore, if this layer is dated to between 5-7 Ma then that is when the prints were made.