High-resolution system time on Windows 8+ and Windows Server 2012+

windows-systime.py

from ctypes import Structure, pointer, windll
from ctypes.wintypes import DWORD, WORD
from datetime import datetime

class FILETIME(Structure):
    _fields_ = [('dwLowDateTime', DWORD),
                ('dwHighDateTime', DWORD)]

_kernel32 = windll.kernel32
GetLastError = _kernel32.GetLastError
SystemTimeToFileTime = _kernel32.SystemTimeToFileTime
try:
    # available in Windows 8+ and Windows Server 2012+
    get_systime = _kernel32.GetSystemTimePreciseAsFileTime
    func_systime = 'GetSystemTimePreciseAsFileTime'
except AttributeError:
    get_systime = _kernel32.GetSystemTimeAsFileTime
    func_systime = 'GetSystemTimeAsFileTime'

def _get_filetime_epoch():
    class SYSTEMTIME(Structure):
        _fields_ = [('wYear', WORD),
                    ('wMonth', WORD),
                    ('wDayOfWeek', WORD),
                    ('wDay', WORD),
                    ('wHour', WORD),
                    ('wMinute', WORD),
                    ('wSecond', WORD),
                    ('wMilliseconds', WORD)]

    st = SYSTEMTIME()
    st.wYear = 1970
    st.wMonth = 1
    st.wDay = 1
    st.wDayOfWeek = 0
    st.wHour = st.wMinute = st.wSecond = st.wMilliseconds = 0
    ft = FILETIME()
    if SystemTimeToFileTime(pointer(st), pointer(ft)):
        return ft.dwHighDateTime * 0x100000000L + ft.dwLowDateTime
    else:
        raise RuntimeError('[LastError SystemTimeToFileTime %d]' %
                           GetLastError())

_filetime_epoch = _get_filetime_epoch()

def now(tz=None):
    ft = FILETIME()
    if not get_systime(pointer(ft)):
        raise RuntimeError('[LastError %s %d]' %
                           (func_systime, GetLastError()))
    ft = ft.dwHighDateTime * 0x100000000L + ft.dwLowDateTime
    usec = (ft - _filetime_epoch) / 10L
    return datetime.fromtimestamp(usec / 1000000.0, tz)

def test_now(f, n):
    print('\n'.join(f().isoformat() for i in xrange(n)))

n = 5
print('datetime.now:')
test_now(datetime.now, n)
print
print(func_systime + ':')
test_now(now, n)

Results:

datetime.now:
2016-01-12T20:18:51.314000
2016-01-12T20:18:51.314000
2016-01-12T20:18:51.314000
2016-01-12T20:18:51.314000
2016-01-12T20:18:51.314000

GetSystemTimePreciseAsFileTime:
2016-01-12T20:18:51.323156
2016-01-12T20:18:51.323178
2016-01-12T20:18:51.323185
2016-01-12T20:18:51.323192
2016-01-12T20:18:51.323198