diff --git a/src/org/omegat/logger.properties b/src/org/omegat/logger.properties
index dbcd046734..dd5cc9a0fb 100644
--- a/src/org/omegat/logger.properties
+++ b/src/org/omegat/logger.properties
@@ -8,16 +8,19 @@ org.omegat.level = ALL
 java.util.logging.ConsoleHandler.level = ALL
 org.omegat.util.logging.OmegaTFileHandler.level = ALL
 
-org.omegat.util.logging.OmegaTLogFormatter.mask=$mark: $level: $text $key
+org.omegat.util.logging.OmegaTLogFormatter.mask=$time: $level: $text $key
 #org.omegat.util.logging.OmegaTLogFormatter.mask=$time: $threadName [$level] $key $text
+#org.omegat.util.logging.OmegaTLogFormatter.mask=$mark: $level: $text $key
 
-org.omegat.util.logging.OmegaTLogFormatter.timeFormat=HH:mm:ss,SSSS
+org.omegat.util.logging.OmegaTLogFormatter.timeFormat=HH:mm:ss.SSS
 
 java.util.logging.ConsoleHandler.formatter = org.omegat.util.logging.OmegaTLogFormatter
 
 org.omegat.util.logging.OmegaTFileHandler.formatter = org.omegat.util.logging.OmegaTLogFormatter
 org.omegat.util.logging.OmegaTFileHandler.size=1048576
 org.omegat.util.logging.OmegaTFileHandler.count=10
+# 20 days * 24 hours * 60 min * 60 sec = 1,020,000 seconds
+org.omegat.util.logging.OmegaTFileHandler.retention=1020000
 
 # precise control of log levels, please change when debug
 org.omegat.core.data.level = INFO
diff --git a/src/org/omegat/util/Log.java b/src/org/omegat/util/Log.java
index b36b2bee9b..4ae381383a 100644
--- a/src/org/omegat/util/Log.java
+++ b/src/org/omegat/util/Log.java
@@ -35,7 +35,6 @@
 import java.io.IOException;
 import java.io.InputStream;
 import java.time.ZonedDateTime;
-import java.time.format.DateTimeFormatter;
 import java.util.Properties;
 import java.util.concurrent.ThreadLocalRandom;
 import java.util.logging.Formatter;
@@ -63,14 +62,14 @@ public final class Log {
 
     // Line mark is day-of-the-year and five-character random number
     private static final String SESSION_ID;
-    private static final String SESSION_START_DATETIME;
+    private static final ZonedDateTime SESSION_START_DATETIME;
 
     private Log() {
     }
 
     static {
         SESSION_ID = String.format("%05d", ThreadLocalRandom.current().nextInt(100000));
-        SESSION_START_DATETIME = DateTimeFormatter.ofPattern("yyyyMMddHHmmss").format(ZonedDateTime.now());
+        SESSION_START_DATETIME = ZonedDateTime.now();
         LOGGER = LoggerFactory.getLogger(ILogger.ROOT_LOGGER_NAME,
                 OStrings.getResourceBundle());
         init();
@@ -80,7 +79,7 @@ public static String getSessionId() {
         return SESSION_ID;
     }
 
-    public static String getSessionStartDateTime() {
+    public static ZonedDateTime getSessionStartDateTime() {
         return SESSION_START_DATETIME;
     }
 
diff --git a/src/org/omegat/util/logging/OmegaTFileHandler.java b/src/org/omegat/util/logging/OmegaTFileHandler.java
index 529448bda2..554fa09ab0 100644
--- a/src/org/omegat/util/logging/OmegaTFileHandler.java
+++ b/src/org/omegat/util/logging/OmegaTFileHandler.java
@@ -34,6 +34,7 @@
 import java.nio.file.Path;
 import java.nio.file.Paths;
 import java.nio.file.attribute.BasicFileAttributes;
+import java.time.format.DateTimeFormatter;
 import java.util.logging.ErrorManager;
 import java.util.logging.Level;
 import java.util.logging.LogManager;
@@ -115,7 +116,7 @@ private void openFiles(final File dir) throws IOException {
         boolean ignored = dir.mkdirs();
         for (int instanceIndex = 0; instanceIndex < 100; instanceIndex++) {
             String fileName = String.format("%s_%s_%s%s", OStrings.getApplicationName(), Log.getSessionId(),
-                    Log.getSessionStartDateTime(),
+                    DateTimeFormatter.ofPattern("yyyyMMdd-HHmmss").format(Log.getSessionStartDateTime()),
                     // Instance index
                     instanceIndex > 0 ? ("-" + instanceIndex) : "");