diff --git a/xen/arch/arm/time.c b/xen/arch/arm/time.c
index 6955b2788f..be54b87438 100644
--- a/xen/arch/arm/time.c
+++ b/xen/arch/arm/time.c
@@ -145,6 +145,8 @@ void __init preinit_xen_time(void)
         panic("Timer: Cannot initialize platform timer\n");
 
     boot_count = get_cycles();
+    smp_wmb();
+    NOW_good = true;
 }
 
 static void __init init_dt_xen_time(void)
diff --git a/xen/arch/riscv/time.c b/xen/arch/riscv/time.c
index 10b7e35f13..602c029641 100644
--- a/xen/arch/riscv/time.c
+++ b/xen/arch/riscv/time.c
@@ -87,6 +87,8 @@ void __init preinit_xen_time(void)
         panic("%s: ACPI isn't supported\n", __func__);
 
     boot_clock_cycles = get_cycles();
+    smp_wmb();
+    NOW_good = true;
 
     /* set_xen_timer must have been set by sbi_init() already */
     ASSERT(set_xen_timer);
diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index bc76e20777..bc0161f7c2 100644
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -2660,6 +2660,8 @@ void __init early_time_init(void)
 
     set_time_scale(&t->tsc_scale, tmp);
     t->stamp.local_tsc = boot_tsc_stamp;
+    barrier();
+    NOW_good = true;
 
     init_percpu_time();
 
diff --git a/xen/common/time.c b/xen/common/time.c
index 04a65f00b3..2b8c98e2de 100644
--- a/xen/common/time.c
+++ b/xen/common/time.c
@@ -22,6 +22,8 @@
 #include <asm/div64.h>
 #include <asm/domain.h>
 
+bool __ro_after_init NOW_good;
+
 /* Nonzero if YEAR is a leap year (every 4 years,
    except every 100th isn't, and every 400th is).  */
 #define __isleap(year) \
diff --git a/xen/drivers/char/console.c b/xen/drivers/char/console.c
index 40355c1d14..fcacf37c52 100644
--- a/xen/drivers/char/console.c
+++ b/xen/drivers/char/console.c
@@ -987,11 +987,11 @@ static void printk_start_of_line(const char *prefix)
         }
         /* fall through */
     case TSM_BOOT:
-        sec = NOW();
-        nsec = do_div(sec, 1000000000);
-
-        if ( sec | nsec )
+        if ( NOW_good )
         {
+            smp_rmb();
+            sec = NOW();
+            nsec = do_div(sec, 1000000000);
             snprintf(tstr, sizeof(tstr), "[%5"PRIu64".%06"PRIu64"] ",
                      sec, nsec / 1000);
             break;
diff --git a/xen/include/xen/time.h b/xen/include/xen/time.h
index e9c0822e6f..4db24617a9 100644
--- a/xen/include/xen/time.h
+++ b/xen/include/xen/time.h
@@ -62,6 +62,12 @@ struct tm wallclock_time(uint64_t *ns);
 /* Chosen so (NOW() + delta) wont overflow without an uptime of 200 years */
 #define STIME_DELTA_MAX ((s_time_t)((uint64_t)~0ULL>>2))
 
+/*
+ * Indicator that the value returned by NOW() is good (earlier invocations may
+ * return zero or very small, merely monotonically increasing values).
+ */
+extern bool NOW_good;
+
 /* Explicitly OR with 1 just in case version number gets out of sync. */
 #define version_update_begin(v) (((v) + 1) | 1)
 #define version_update_end(v)   ((v) + 1)