diff --git a/README.md b/README.md
index 8776157..bdfc19d 100644
--- a/README.md
+++ b/README.md
@@ -76,9 +76,9 @@ swift run monitord --retention 7d --dir /tmp/logs      # rotating CSV logger
 ```
 
 `monitord` is the logger: it samples every metric on the same clock and writes
-rotating, human-readable CSV — one file per day, hostname in the filename and as
-a column, timestamps in ISO8601 and epoch millis, temperatures in both °C and
-°F. Run it as a launchd `LaunchAgent` to log for days.
+rotating, human-readable CSV — one file per run, hostname in the filename and
+as a column, timestamps in ISO8601 and epoch millis, temperatures in both °C
+and °F. Run it as a launchd `LaunchAgent` to log for days.
 
 With no options it logs at 1s with 1d retention to `~/Library/Logs/monitor`.
 The release zip ships a standalone `monitord` binary alongside `monitor.app`, so
diff --git a/Sources/MonitorCore/LogRetention.swift b/Sources/MonitorCore/LogRetention.swift
index 589539f..df8f6f3 100644
--- a/Sources/MonitorCore/LogRetention.swift
+++ b/Sources/MonitorCore/LogRetention.swift
@@ -42,16 +42,17 @@ public enum LogRetention: String, CaseIterable, Sendable {
         return calendar.date(from: components)?.timeIntervalSince1970 ?? timestamp
     }
 
-    /// The day a file covers, read back from its name. The date is the last
-    /// component of the name, so a hostname that itself contains dashes or dots
-    /// cannot confuse the parse.
+    /// The day a file covers, read back from its name. The date and time are
+    /// the last components of the name, so a hostname that itself contains
+    /// dashes or dots cannot confuse the parse. The time is ignored: the period
+    /// is the start of the day, so retention still deletes whole days.
     public static func period(from filename: String) -> TimeInterval? {
         let base = filename.hasSuffix(".csv") ? String(filename.dropLast(4)) : filename
         let formatter = DateFormatter()
         formatter.timeZone = .current
-        formatter.dateFormat = "yyyy_MM_dd"
-        guard base.count >= 10 else { return nil }
-        guard let date = formatter.date(from: String(base.suffix(10))) else { return nil }
-        return date.timeIntervalSince1970
+        formatter.dateFormat = "yyyy_MM_dd_HH_mm_ss"
+        guard base.count >= 19 else { return nil }
+        guard let date = formatter.date(from: String(base.suffix(19))) else { return nil }
+        return period(for: date.timeIntervalSince1970)
     }
 }
diff --git a/Sources/MonitorLog/CSVLogSink.swift b/Sources/MonitorLog/CSVLogSink.swift
index c978b39..7b17017 100644
--- a/Sources/MonitorLog/CSVLogSink.swift
+++ b/Sources/MonitorLog/CSVLogSink.swift
@@ -3,10 +3,10 @@ import MonitorCore
 
 /// Writes sampled batches to rotating CSV files.
 ///
-/// One file per day, named `sensors.<host>.<date>.csv` so files from several
-/// machines sharing a directory do not clobber. The header is written when a
-/// file is first created; a file reopened after a restart appends without
-/// repeating it.
+/// One file per run, named `sensors.<host>.<date>_<time>.csv` so files from
+/// several machines sharing a directory do not clobber, and a restart does not
+/// append to the previous run's file. The header is written when a file is
+/// first created.
 ///
 /// Retention deletes whole files whose period is older than the window. It runs
 /// on a slow timer, not on every write, so a log that runs for days does not pay
@@ -40,7 +40,7 @@ public actor CSVLogSink: SampleSink {
         let period = LogRetention.period(for: batch.timestamp)
         if current?.period != period {
             closeCurrent()
-            open(period: period)
+            open(period: period, at: batch.timestamp)
         }
         guard let handle = current?.handle else { return }
         let values = Dictionary(
@@ -66,8 +66,8 @@ public actor CSVLogSink: SampleSink {
 
     // MARK: - Files
 
-    private func open(period: TimeInterval) {
-        let url = directory.appendingPathComponent(filename(for: period))
+    private func open(period: TimeInterval, at timestamp: TimeInterval) {
+        let url = directory.appendingPathComponent(filename(at: timestamp))
         let isNew = !FileManager.default.fileExists(atPath: url.path)
         if isNew {
             FileManager.default.createFile(atPath: url.path, contents: nil)
@@ -86,11 +86,11 @@ public actor CSVLogSink: SampleSink {
         current = nil
     }
 
-    private func filename(for period: TimeInterval) -> String {
-        let date = Date(timeIntervalSince1970: period)
+    private func filename(at timestamp: TimeInterval) -> String {
+        let date = Date(timeIntervalSince1970: timestamp)
         let formatter = DateFormatter()
         formatter.timeZone = .current
-        formatter.dateFormat = "yyyy_MM_dd"
+        formatter.dateFormat = "yyyy_MM_dd_HH_mm_ss"
         return "sensors.\(Self.sanitized(hostname)).\(formatter.string(from: date)).csv"
     }
 
diff --git a/Sources/monitord/main.swift b/Sources/monitord/main.swift
index dc27d60..2a77ffd 100644
--- a/Sources/monitord/main.swift
+++ b/Sources/monitord/main.swift
@@ -23,10 +23,11 @@ OPTIONS
   --interval <sec>     sampling interval (default 1.0)
 
 NOTES
-  Files are named sensors.<host>.<date>.csv, one per day, so files from several
-  machines sharing a directory do not clobber. The hostname is lowercased and
-  any character outside [a-z0-9-_] becomes an underscore. Timestamps are ISO8601
-  in UTC plus epoch millis. Temperatures appear in both degrees C and degrees F.
+  Files are named sensors.<host>.<date>_<time>.csv, one per run, so files from
+  several machines sharing a directory do not clobber, and a restart does not
+  append to the previous run's file. The hostname is lowercased and any
+  character outside [a-z0-9-_] becomes an underscore. Timestamps are ISO8601 in
+  UTC plus epoch millis. Temperatures appear in both degrees C and degrees F.
 """
 
 let arguments = Array(CommandLine.arguments.dropFirst())
diff --git a/Tests/MonitorCoreTests/LogRetentionTests.swift b/Tests/MonitorCoreTests/LogRetentionTests.swift
index c53680b..a9e435e 100644
--- a/Tests/MonitorCoreTests/LogRetentionTests.swift
+++ b/Tests/MonitorCoreTests/LogRetentionTests.swift
@@ -34,7 +34,7 @@ struct LogRetentionTests {
             month: 9,
             day: 4
         )))
-        let parsed = LogRetention.period(from: "sensors.my-host.2026_09_04.csv")
+        let parsed = LogRetention.period(from: "sensors.my-host.2026_09_04_12_34_56.csv")
         #expect(parsed == day.timeIntervalSince1970)
     }
 }
diff --git a/Tests/MonitorLogTests/CSVLogSinkTests.swift b/Tests/MonitorLogTests/CSVLogSinkTests.swift
index 5b03dba..04c5ceb 100644
--- a/Tests/MonitorLogTests/CSVLogSinkTests.swift
+++ b/Tests/MonitorLogTests/CSVLogSinkTests.swift
@@ -95,6 +95,30 @@ struct CSVLogSinkTests {
         #expect(files[0].lastPathComponent.hasPrefix("sensors.macbook-pro_local."))
     }
 
+    @Test func filenameIncludesTimeOfDay() async throws {
+        let dir = try tempDir()
+        defer { try? FileManager.default.removeItem(at: dir) }
+        let sink = try CSVLogSink(
+            directory: dir, hostname: "myhost", retention: .sevenDays, descriptors: descriptors
+        )
+        let t = Date(timeIntervalSince1970: 1_750_000_000).timeIntervalSince1970
+        await sink.receive(SampleBatch(
+            timestamp: t,
+            values: [MetricID("sensor.temperature.cpu"): 45.0]
+        ))
+        await sink.close()
+
+        let files = try FileManager.default.contentsOfDirectory(
+            at: dir,
+            includingPropertiesForKeys: nil
+        )
+        let formatter = DateFormatter()
+        formatter.timeZone = .current
+        formatter.dateFormat = "yyyy_MM_dd_HH_mm_ss"
+        let expected = "sensors.myhost.\(formatter.string(from: Date(timeIntervalSince1970: t))).csv"
+        #expect(files[0].lastPathComponent == expected)
+    }
+
     @Test func rollsOverOnCadenceBoundary() async throws {
         let dir = try tempDir()
         defer { try? FileManager.default.removeItem(at: dir) }