say timestamp not date

test/README.md

@@ -11,7 +11,7 @@ $ python3 sync_test.py
 
 Once started sync_handshake.py is creating a trace of all sent messages in
 TestInbox. There's a file named Laptop or Phone, respectively, which is working
-as a marker; the modification date of this file is showing the mails being
+as a marker; the modification timestamp of this file is showing the mails being
 considered as “already received”. If you delete the file and start i.e.
 
 $ cd Phone
@@ -19,5 +19,5 @@ $ rm ../TestInbox/Phone
 $ HOME=$PWD lldb python3 --  ../sync_handshake.py -e Phone
 
 Then this side is doing a replay in the debugger.  Using touch to set a
-different date on the marker will only partly replay.
+different timestamp on the marker will only partly replay.