Replace `start` with `ticket_creation`

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_tls13_server.c b/library/ssl_tls13_server.c
index 7b8cc6e..744e984 100644
--- a/library/ssl_tls13_server.c
+++ b/library/ssl_tls13_server.c
@@ -193,15 +193,15 @@
 #if defined(MBEDTLS_HAVE_TIME)
     now = mbedtls_ms_time();
 
-    if (now < session->start) {
+    if (now < session->ticket_creation) {
         MBEDTLS_SSL_DEBUG_MSG(
             3, ("Invalid ticket start time ( now = %" MBEDTLS_PRINTF_MS_TIME
                 ", start = %" MBEDTLS_PRINTF_MS_TIME " )",
-                now, session->start));
+                now, session->ticket_creation));
         goto exit;
     }
 
-    server_age = now - session->start;
+    server_age = now - session->ticket_creation;
 
     /* RFC 8446 section 4.6.1
      *
@@ -2880,7 +2880,7 @@
     MBEDTLS_SSL_DEBUG_MSG(2, ("=> prepare NewSessionTicket msg"));
 
 #if defined(MBEDTLS_HAVE_TIME)
-    session->start = mbedtls_ms_time();
+    session->ticket_creation = mbedtls_ms_time();
 #endif
 
     /* Set ticket_flags depends on the advertised psk key exchange mode */