diff --git a/include/libswiftnav/time.h b/include/libswiftnav/time.h
index 4fb45fd8..87641735 100644
--- a/include/libswiftnav/time.h
+++ b/include/libswiftnav/time.h
@@ -77,6 +77,7 @@
 #define GPS_EPOCH 315964800
 
 #define WN_UNKNOWN -1
+#define TOW_UNKNOWN -1
 
 /** Structure representing a GPS time. */
 typedef struct __attribute__((packed)) {
@@ -84,6 +85,8 @@ typedef struct __attribute__((packed)) {
   s16 wn;     /**< GPS week number. */
 } gps_time_t;
 
+#define GPS_TIME_UNKNOWN ((gps_time_t){TOW_UNKNOWN, WN_UNKNOWN})
+
 void normalize_gps_time(gps_time_t *t_gps);
 
 time_t gps2time(const gps_time_t *t);
diff --git a/src/time.c b/src/time.c
index 00181b71..77cbe741 100644
--- a/src/time.c
+++ b/src/time.c
@@ -144,11 +144,14 @@ gps_time_t glo_time2gps_time(u16 nt, u8 n4, s8 h, s8 m, s8 s)
   u8 j = 0;
   u16 day_of_year = 0;
   u32 glo_year = 0;
-  gps_time_t gps_t = {0, 0};
+  gps_time_t gps_t = GPS_TIME_UNKNOWN;
 
-  assert(n4 >= N4_MIN && n4 <= N4_MAX);
+  if (n4 < N4_MIN || n4 > N4_MAX)
+    return GPS_TIME_UNKNOWN;
 
-  if (nt >= GLO_NT_0_FLOOR && nt <= GLO_NT_0_CEILING) {
+  if (nt < GLO_NT_0_FLOOR)
+    return GPS_TIME_UNKNOWN;
+  else if (nt <= GLO_NT_0_CEILING) {
     j = 1;
     day_of_year = nt;
   }
@@ -165,7 +168,7 @@ gps_time_t glo_time2gps_time(u16 nt, u8 n4, s8 h, s8 m, s8 s)
     day_of_year = nt - LEAP_YEAR_DAYS - YEAR_DAYS * 2;
   }
   else
-    assert(0 && "invalid nt");
+    return GPS_TIME_UNKNOWN;
 
   glo_year = 1996 + 4 * (n4 - 1) + (j - 1);
 
diff --git a/tests/check_time.c b/tests/check_time.c
index c9a93cfb..d50a4653 100644
--- a/tests/check_time.c
+++ b/tests/check_time.c
@@ -201,6 +201,14 @@ START_TEST(test_glo_time2gps_time)
     /* GLO time 31st Dec 2011 12:12:12 */
     {.nt = 1461, .n4 = 4, .h = 12, .m = 12, .s = 12,
       .ret = {.wn = 1668, .tow = 551549}},
+    {.nt = 0, .n4 = 4, .h = 12, .m = 12, .s = 12,
+      .ret = GPS_TIME_UNKNOWN},
+    {.nt = 1462, .n4 = 4, .h = 12, .m = 12, .s = 12,
+      .ret = GPS_TIME_UNKNOWN},
+    {.nt = 1461, .n4 = 0, .h = 12, .m = 12, .s = 12,
+      .ret = GPS_TIME_UNKNOWN},
+    {.nt = 1461, .n4 = 32, .h = 12, .m = 12, .s = 12,
+      .ret = GPS_TIME_UNKNOWN},
   };
   for (size_t i = 0;
        i < sizeof(testcases) / sizeof(struct glo_time2gps_time_testcase);