diff --git a/introspection/org.freedesktop.ModemManager1.Modem.Time.xml b/introspection/org.freedesktop.ModemManager1.Modem.Time.xml
index 5278c774..0558f029 100644
--- a/introspection/org.freedesktop.ModemManager1.Modem.Time.xml
+++ b/introspection/org.freedesktop.ModemManager1.Modem.Time.xml
@@ -20,9 +20,11 @@
 
     <!--
         GetNetworkTime:
-        @time: If the network time is known, a string containing date and time in ISO 8601 format. If the network time is unknown, the empty string.
+        @time: If the network time is known, a string containing local date,
+        time, and (if available) UTC offset in ISO 8601 format. If the network
+        time is unknown, the empty string.
 
-        Gets the current network time.
+        Gets the current network time in local time.
 
         This method will only work if the modem tracks, or can request, the
         current network time; it will not attempt to use previously-received
diff --git a/plugins/icera/mm-broadband-modem-icera.c b/plugins/icera/mm-broadband-modem-icera.c
index bedf91ce..e71557c4 100644
--- a/plugins/icera/mm-broadband-modem-icera.c
+++ b/plugins/icera/mm-broadband-modem-icera.c
@@ -1493,7 +1493,9 @@ parse_tlts_query_reply (const gchar *response,
     gint second;
     gchar sign;
     gint offset;
+    GDateTime *utc, *adjusted;
 
+    /* TLTS reports UTC time with the TZ offset to *local* time */
     response = mm_strip_tag (response, "*TLTS: ");
     if (sscanf (response,
                 "\"%02d/%02d/%02d,%02d:%02d:%02d%c%02d\"",
@@ -1504,42 +1506,70 @@ parse_tlts_query_reply (const gchar *response,
                 &minute,
                 &second,
                 &sign,
-                &offset) == 8) {
-        /* Offset comes in 15-min intervals */
-        offset *= 15;
-        /* Apply sign to offset */
-        if (sign == '-')
-            offset *= -1;
-
-        /* If asked for it, build timezone information */
-        if (tz) {
-            *tz = mm_network_timezone_new ();
-            mm_network_timezone_set_offset (*tz, offset);
-        }
-
-        if (iso8601) {
-            /* Icera modems only report a 2-digit year, while ISO-8601 requires
-             * a 4-digit year.  Assume 2000.
-             */
-            if (year < 100)
-                year += 2000;
-
-            /* don't give tz info in the date/time string, we have another
-             * property for that */
-            *iso8601 = g_strdup_printf ("%04d/%02d/%02d %02d:%02d:%02d",
-                                        year, month, day,
-                                        hour, minute, second);
-        }
-
-        return TRUE;
+                &offset) != 8) {
+        g_set_error (error,
+                     MM_CORE_ERROR,
+                     MM_CORE_ERROR_FAILED,
+                     "Unknown *TLTS response: %s",
+                     response);
+        return FALSE;
     }
 
-    g_set_error (error,
-                 MM_CORE_ERROR,
-                 MM_CORE_ERROR_FAILED,
-                 "Unknown *TLTS response: %s",
-                 response);
-    return FALSE;
+    /* Icera modems only report a 2-digit year, while ISO-8601 requires
+     * a 4-digit year.  Assume 2000.
+     */
+    if (year < 100)
+        year += 2000;
+
+    /* Offset comes in 15-min units */
+    offset *= 15;
+    /* Apply sign to offset;  */
+    if (sign == '-')
+        offset *= -1;
+
+    utc = g_date_time_new_utc (year, month, day, hour, minute, second);
+    if (!utc) {
+        g_set_error (error,
+                     MM_CORE_ERROR,
+                     MM_CORE_ERROR_FAILED,
+                     "Invalid *TLTS date/time: %s",
+                     response);
+        return FALSE;
+    }
+
+    /* Convert UTC time to local time by adjusting by the timezone offset */
+    adjusted = g_date_time_add_minutes (utc, offset);
+    g_date_time_unref (utc);
+    if (!adjusted) {
+        g_set_error (error,
+                     MM_CORE_ERROR,
+                     MM_CORE_ERROR_FAILED,
+                     "Failed to convert modem time to local time (offset %d)",
+                     offset);
+        return FALSE;
+    }
+
+    /* Convert offset from minutes-to-UTC to minutes-from-UTC */
+    offset *= -1;
+
+    if (tz) {
+        *tz = mm_network_timezone_new ();
+        mm_network_timezone_set_offset (*tz, offset);
+    }
+
+    if (iso8601) {
+        *iso8601 = mm_new_iso8601_time (g_date_time_get_year (adjusted),
+                                        g_date_time_get_month (adjusted),
+                                        g_date_time_get_day_of_month (adjusted),
+                                        g_date_time_get_hour (adjusted),
+                                        g_date_time_get_minute (adjusted),
+                                        g_date_time_get_second (adjusted),
+                                        TRUE,
+                                        offset);
+    }
+
+    g_date_time_unref (adjusted);
+    return TRUE;
 }
 
 static MMNetworkTimezone *
diff --git a/plugins/novatel/mm-broadband-modem-novatel.c b/plugins/novatel/mm-broadband-modem-novatel.c
index f0376752..3d84509b 100644
--- a/plugins/novatel/mm-broadband-modem-novatel.c
+++ b/plugins/novatel/mm-broadband-modem-novatel.c
@@ -1012,9 +1012,8 @@ parse_nwltime_reply (const char *response,
             mm_get_uint_from_match_info (match_info, 6, &second) &&
             mm_get_int_from_match_info (match_info, 8, &utc_offset)) {
 
-            /* Return ISO-8601 format date/time string */
-            result = g_strdup_printf ("%04d/%02d/%02d %02d:%02d:%02d",
-                                      year, month, day, hour, minute, second);
+            result = mm_new_iso8601_time (year, month, day, hour, minute, second,
+                                          TRUE, utc_offset * 60);
             if (out_tz) {
                 *out_tz = mm_network_timezone_new ();
                 mm_network_timezone_set_offset (*out_tz, utc_offset * 60);
diff --git a/plugins/sierra/mm-broadband-modem-sierra.c b/plugins/sierra/mm-broadband-modem-sierra.c
index 301f41eb..7cb8e6cf 100644
--- a/plugins/sierra/mm-broadband-modem-sierra.c
+++ b/plugins/sierra/mm-broadband-modem-sierra.c
@@ -1337,9 +1337,7 @@ parse_time (const gchar *response,
             mm_get_uint_from_match_info (match_info, 4, &hour) &&
             mm_get_uint_from_match_info (match_info, 5, &minute) &&
             mm_get_uint_from_match_info (match_info, 6, &second)) {
-            /* Return ISO-8601 format date/time string */
-            result = g_strdup_printf ("%04d/%02d/%02d %02d:%02d:%02d",
-                                      year, month, day, hour, minute, second);
+            result = mm_new_iso8601_time (year, month, day, hour, minute, second, FALSE, 0);
         } else {
             g_set_error (error,
                          MM_CORE_ERROR,
diff --git a/src/mm-modem-helpers.c b/src/mm-modem-helpers.c
index 09bb71d7..44a00a53 100644
--- a/src/mm-modem-helpers.c
+++ b/src/mm-modem-helpers.c
@@ -216,6 +216,38 @@ mm_filter_current_bands (const GArray *supported_bands,
 
 /*****************************************************************************/
 
+gchar *
+mm_new_iso8601_time (guint year,
+                     guint month,
+                     guint day,
+                     guint hour,
+                     guint minute,
+                     guint second,
+                     gboolean have_offset,
+                     gint offset_minutes)
+{
+    GString *str;
+
+    str = g_string_sized_new (30);
+    g_string_append_printf (str, "%04d-%02d-%02dT%02d:%02d:%02d",
+                            year, month, day, hour, minute, second);
+    if (have_offset) {
+        if (offset_minutes >=0 ) {
+            g_string_append_printf (str, "+%02d:%02d",
+                                    offset_minutes / 60,
+                                    offset_minutes % 60);
+        } else {
+            offset_minutes *= -1;
+            g_string_append_printf (str, "-%02d:%02d",
+                                    offset_minutes / 60,
+                                    offset_minutes % 60);
+        }
+    }
+    return g_string_free (str, FALSE);
+}
+
+/*****************************************************************************/
+
 /* +CREG: <stat>                      (GSM 07.07 CREG=1 unsolicited) */
 #define CREG1 "\\+(CREG|CGREG|CEREG):\\s*0*([0-9])"
 
diff --git a/src/mm-modem-helpers.h b/src/mm-modem-helpers.h
index eb02337e..5a8ba493 100644
--- a/src/mm-modem-helpers.h
+++ b/src/mm-modem-helpers.h
@@ -64,6 +64,15 @@ guint mm_netmask_to_cidr (const gchar *netmask);
 GArray *mm_filter_current_bands (const GArray *supported_bands,
                                  const GArray *current_bands);
 
+gchar *mm_new_iso8601_time (guint year,
+                            guint month,
+                            guint day,
+                            guint hour,
+                            guint minute,
+                            guint second,
+                            gboolean have_offset,
+                            gint offset_minutes);
+
 /*****************************************************************************/
 /* 3GPP specific helpers and utilities */
 /*****************************************************************************/