diff --git a/src/bin/ent/main.rs b/src/bin/ent/main.rs
index 494b1ba..ac55045 100644
--- a/src/bin/ent/main.rs
+++ b/src/bin/ent/main.rs
@@ -86,6 +86,12 @@ enum Commands {
         #[arg(allow_hyphen_values = true)]
         tag: Option<String>,
     },
+
+    /// Get or set the `done_time` of the Issue.
+    DoneTime {
+        issue_id: String,
+        done_time: Option<String>,
+    },
 }
 
 fn handle_command(
@@ -263,6 +269,9 @@ fn handle_command(
                     println!("issue {}", issue_id);
                     println!("author: {}", issue.author);
                     println!("creation_time: {}", issue.creation_time);
+                    if let Some(done_time) = &issue.done_time {
+                        println!("done_time: {}", done_time);
+                    }
                     println!("state: {:?}", issue.state);
                     if let Some(dependencies) = &issue.dependencies {
                         println!("dependencies: {:?}", dependencies);
@@ -448,6 +457,38 @@ fn handle_command(
                 }
             }
         }
+
+        Commands::DoneTime {
+            issue_id,
+            done_time,
+        } => {
+            let issues = entomologist::database::read_issues_database(issues_database_source)?;
+            let Some(issue) = issues.issues.get(issue_id) else {
+                return Err(anyhow::anyhow!("issue {} not found", issue_id));
+            };
+            match done_time {
+                Some(done_time) => {
+                    // Add or remove tag.
+                    let issues_database = entomologist::database::make_issues_database(
+                        issues_database_source,
+                        entomologist::database::IssuesDatabaseAccess::ReadWrite,
+                    )?;
+                    let mut issues =
+                        entomologist::issues::Issues::new_from_dir(&issues_database.dir)?;
+                    let Some(issue) = issues.get_mut_issue(issue_id) else {
+                        return Err(anyhow::anyhow!("issue {} not found", issue_id));
+                    };
+                    let done_time = chrono::DateTime::parse_from_rfc3339(done_time)
+                        .unwrap()
+                        .with_timezone(&chrono::Local);
+                    issue.set_done_time(done_time)?;
+                }
+                None => match &issue.done_time {
+                    Some(done_time) => println!("done_time: {}", done_time),
+                    None => println!("None"),
+                },
+            };
+        }
     }
 
     Ok(())
diff --git a/src/issue.rs b/src/issue.rs
index 927eb7f..bc3d959 100644
--- a/src/issue.rs
+++ b/src/issue.rs
@@ -283,7 +283,8 @@ impl Issue {
         }
     }
 
-    /// Change the State of the Issue.
+    /// Change the State of the Issue.  If the new state is `Done`,
+    /// set the Issue `done_time`.  Commits.
     pub fn set_state(&mut self, new_state: State) -> Result<(), IssueError> {
         let old_state = self.state.clone();
         let mut state_filename = std::path::PathBuf::from(&self.dir);
@@ -296,6 +297,9 @@ impl Issue {
             old_state,
             new_state,
         ))?;
+        if new_state == State::Done {
+            self.set_done_time(chrono::Local::now())?;
+        }
         Ok(())
     }
 
@@ -307,6 +311,24 @@ impl Issue {
         Ok(())
     }
 
+    /// Set the `done_time` of the Issue.  Commits.
+    pub fn set_done_time(
+        &mut self,
+        done_time: chrono::DateTime<chrono::Local>,
+    ) -> Result<(), IssueError> {
+        let mut done_time_filename = std::path::PathBuf::from(&self.dir);
+        done_time_filename.push("done_time");
+        let mut done_time_file = std::fs::File::create(&done_time_filename)?;
+        write!(done_time_file, "{}", done_time.to_rfc3339())?;
+        self.done_time = Some(done_time.clone());
+        self.commit(&format!(
+            "set done-time of issue {} to {}",
+            self.dir.file_name().unwrap().to_string_lossy(),
+            done_time,
+        ))?;
+        Ok(())
+    }
+
     /// Set the Assignee of an Issue.
     pub fn set_assignee(&mut self, new_assignee: &str) -> Result<(), IssueError> {
         let old_assignee = match &self.assignee {