diff --git a/esphome/components/syslog/esphome_syslog.cpp b/esphome/components/syslog/esphome_syslog.cpp
index f5c20c891..851fb30c2 100644
--- a/esphome/components/syslog/esphome_syslog.cpp
+++ b/esphome/components/syslog/esphome_syslog.cpp
@@ -34,7 +34,15 @@ void Syslog::log_(const int level, const char *tag, const char *message, size_t
     severity = LOG_LEVEL_TO_SYSLOG_SEVERITY[level];
   }
   int pri = this->facility_ * 8 + severity;
-  auto timestamp = this->time_->now().strftime("%b %e %H:%M:%S");
+  auto now = this->time_->now();
+  std::string timestamp;
+  if (now.is_valid()) {
+    timestamp = now.strftime("%b %e %H:%M:%S");
+  } else {
+    // RFC 5424: A syslog application MUST use the NILVALUE as TIMESTAMP if the syslog application is incapable of
+    //           obtaining system time.
+    timestamp = "-";
+  }
   size_t len = message_len;
   // remove color formatting
   if (this->strip_ && message[0] == 0x1B && len > 11) {