Given the attached source, I can produce these results:

[stefan@localhost ~]$ python ziptest.py 
Start 10:05:54
ZIP stored mtime: (2008, 6, 29, 10, 5, 54)
Original mtime: (2008, 6, 29, 10, 5, 54)
Duration 0.00291705131531
Stop 10:05:54
------------------------------
Start 10:05:55
ZIP stored mtime: (2008, 6, 29, 10, 5, 54)
Original mtime: (2008, 6, 29, 10, 5, 55)
Duration 0.00302505493164
Stop 10:05:55
------------------------------
Start 10:05:56
ZIP stored mtime: (2008, 6, 29, 10, 5, 56)
Original mtime: (2008, 6, 29, 10, 5, 56)
Duration 0.00260186195374
Stop 10:05:56
------------------------------
Start 10:05:57
ZIP stored mtime: (2008, 6, 29, 10, 5, 56)
Original mtime: (2008, 6, 29, 10, 5, 57)
Duration 0.00173997879028
Stop 10:05:57
------------------------------
Start 10:05:58
ZIP stored mtime: (2008, 6, 29, 10, 5, 58)
Original mtime: (2008, 6, 29, 10, 5, 58)
Duration 0.00218915939331
Stop 10:05:58
------------------------------
Start 10:05:59
ZIP stored mtime: (2008, 6, 29, 10, 5, 58)
Original mtime: (2008, 6, 29, 10, 5, 59)
Duration 0.00273489952087
Stop 10:05:59
------------------------------
Start 10:06:00
ZIP stored mtime: (2008, 6, 29, 10, 6, 0)
Original mtime: (2008, 6, 29, 10, 6, 0)
Duration 0.00237393379211
Stop 10:06:00
------------------------------
Start 10:06:01
ZIP stored mtime: (2008, 6, 29, 10, 6, 0)
Original mtime: (2008, 6, 29, 10, 6, 1)
Duration 0.00279211997986
Stop 10:06:01
------------------------------
Start 10:06:02
ZIP stored mtime: (2008, 6, 29, 10, 6, 2)
Original mtime: (2008, 6, 29, 10, 6, 2)
Duration 0.00237607955933
Stop 10:06:02
------------------------------
Start 10:06:03
ZIP stored mtime: (2008, 6, 29, 10, 6, 2)
Original mtime: (2008, 6, 29, 10, 6, 3)
Duration 0.0018618106842
Stop 10:06:03
------------------------------


I also printed the duration to see if it passes from one second to another.
It doesn't.
The behaviour is the same with ZIP_STORED as storage type.