When was Hamlet set?
Your question was pretty confusing, since you ask what seems to be a relatively straightforward question about time period in which Hamlet was set, yet you follow this with a mention of post-structuralism in your clarification and tags. I fail to see the connection between time period in which Shakespeare might have set Hamlet and post-structural analysis of the play, since post-structuralism was a 20th century movement in literary criticism. Please re-frame your question about post-structuralism as it relates to Hamlet and then post it for answer.
Meanwhile, here is my response to your question, "When was Hamlet set?"
Shakespeare makes it clear that this play is set in Denmark, but he does not specify time period. By examining the available source material that scholars imagine Shakespeare might have used to create the plot of Hamlet -- and a significant potential source, the Ur-Hamlet, is lost -- you might be able to create a guess about what time period these characters might originally have been created to exist in.
However, Shakespeare was never concerned with time period in staging his plays. All of his plays (including the ones "set" in ancient times like Julius Caesar, Antony and Cleopatra, A Midsummer Night's Dream, King Lear and Pericles, to name a few) were costumed and presented as if they were taking place right in the "modern" Elizabethan world in which Shakespeare lived and worked. His characters also spoke in the ways that people in Shakespeare's society spoke, no matter the place or time or culture they were meant to exist in.
So, no matter what you might unearth about the actual "Prince of Denmark" and when he might have lived, you can be sure that, for Shakespeare, the time was "now," and the dress and conversation was 100% "contemporary."
For more on the historical background of the source material Shakespeare may have used to create Hamlet and the use of "modern" costuming at Shakespeare's Globe, please follow the links below.