The Mark on the Wall Lesson Plan

When is this story set? What is the timeline of the narration? What textual evidence is there for this?

"The Mark on the Wall" was published in 1917, during the First World War. This much is clear, but the timeline of the story itself is debatable. The narration opens in the past tense, leaving readers to believe the narrator is reflecting on a past event, i.e when they first noticed the mark on the wall. However, the use of the word "first" indicates that there may have been further sightings and musings of the mark on the wall. Yet, if the mark was determined to be a snail that first sighting, wouldn't the other person have removed it when they left to buy the...