diff --git a/src/zlib_file.cpp b/src/zlib_file.cpp
index 042dafb..73695b0 100644
--- a/src/zlib_file.cpp
+++ b/src/zlib_file.cpp
@@ -41,9 +41,17 @@
 {
     int rc;
 
-    // Write time stamp
-    rc = gzprintf(fd, "[ %02i:%02i:%02i ] ", timeStamp.tm_hour,
-                  timeStamp.tm_min, timeStamp.tm_sec);
+    // Write time stamp.
+    // "tm_gmtoff" is the number of seconds east of UTC, so we need to calculate
+    // timezone offset. For example, for U.S. Eastern Standard Time, the value
+    // is -18000 = -5*60*60."
+
+    rc = gzprintf(fd, "[ %i-%02i-%02iT%02i:%02i:%02i%+03ld:%02ld ] ",
+                  timeStamp.tm_year + 1900, timeStamp.tm_mon + 1,
+                  timeStamp.tm_mday, timeStamp.tm_hour, timeStamp.tm_min,
+                  timeStamp.tm_sec, timeStamp.tm_gmtoff / (60 * 60),
+                  abs(timeStamp.tm_gmtoff % (60 * 60)) / 60);
+
     if (rc <= 0)
     {
         throw ZlibException(ZlibException::write, rc, fd, fileName);
diff --git a/test/zlib_file_test.cpp b/test/zlib_file_test.cpp
index ef49a6f..65f2206 100644
--- a/test/zlib_file_test.cpp
+++ b/test/zlib_file_test.cpp
@@ -25,9 +25,13 @@
     file.close();
 
     char expect[64];
-    const int len = snprintf(expect, sizeof(expect), "[ %02i:%02i:%02i ] %s\n",
-                             localTime.tm_hour, localTime.tm_min,
-                             localTime.tm_sec, msg.c_str());
+    const int len =
+        snprintf(expect, sizeof(expect),
+                 "[ %i-%02i-%02iT%02i:%02i:%02i%+03ld:%02ld ] %s\n",
+                 localTime.tm_year + 1900, localTime.tm_mon + 1,
+                 localTime.tm_mday, localTime.tm_hour, localTime.tm_min,
+                 localTime.tm_sec, localTime.tm_gmtoff / (60 * 60),
+                 abs(localTime.tm_gmtoff % (60 * 60)) / 60, msg.c_str());
 
     gzFile fd = gzopen(path.c_str(), "r");
     ASSERT_TRUE(fd);