diff --git a/src/calibre/devices/usbms/driver.py b/src/calibre/devices/usbms/driver.py
index d8c8ada680..e56cdb3381 100644
--- a/src/calibre/devices/usbms/driver.py
+++ b/src/calibre/devices/usbms/driver.py
@@ -23,15 +23,13 @@ from calibre.devices.usbms.books import BookList, Book
 from calibre.ebooks.metadata.book.json_codec import JsonCodec
 from polyglot.builtins import itervalues, unicode_type, string_or_bytes, zip
 
-BASE_TIME = None
 
-
-def debug_print(*args):
-    global BASE_TIME
-    if BASE_TIME is None:
-        BASE_TIME = time.time()
+def debug_print(*args, **kw):
+    base_time = getattr(debug_print, 'base_time', None)
+    if base_time is None:
+        debug_print.base_time = base_time = time.monotonic()
     if DEBUG:
-        prints('DEBUG: %6.1f'%(time.time()-BASE_TIME), *args)
+        prints('DEBUG: %6.1f'%(time.monotonic()-base_time), *args, **kw)
 
 
 def safe_walk(top, topdown=True, onerror=None, followlinks=False, maxdepth=128):