Selecting a subset of a Pandas DataFrame indexed by DatetimeIndex with a list of TimeStamps

I have a large Pandas DataFrame

<class 'pandas.core.frame.DataFrame'>
DatetimeIndex: 3425100 entries, 2011-12-01 00:00:00 to 2011-12-31 23:59:59
Data columns:
sig_qual    3425100  non-null values
heave       3425100  non-null values
north       3425099  non-null values
west        3425097  non-null values
dtypes: float64(4)

I select a subset of that DataFrame using .ix[start_datetime:end_datetime] and I pass this to a peakdetect function which returns the index and value of the local maxima and minima in two seperate lists. I extract the index position of the maxima and using DataFrame.index I get a list of pandas TimeStamps.

I then attempt to extract the relevant subset of the large DataFrame by passing the list of TimeStamps to .ix[] but it always seems to return an empty DataFrame. I can loop over the list of TimeStamps and get the relevant rows from the DataFrame but this is a lengthy process and I thought that ix[] should accept a list of values according to the docs? (Although I see that the example for Pandas 0.7 uses a numpy.ndarray of numpy.datetime64)

Update: A small 8 second subset of the DataFrame is selected below, # lines show some of the values:

y = raw_disp['heave'].ix[datetime(2011,12,30,0,0,0):datetime(2011,12,30,0,0,8)]
#csv representation of y time-series 
2011-12-30 00:00:00,-310.0
2011-12-30 00:00:01,-238.0
2011-12-30 00:00:01.500000,-114.0
2011-12-30 00:00:02.500000,60.0
2011-12-30 00:00:03,185.0
2011-12-30 00:00:04,259.0
2011-12-30 00:00:04.500000,231.0
2011-12-30 00:00:05.500000,139.0
2011-12-30 00:00:06.500000,55.0
2011-12-30 00:00:07,-49.0
2011-12-30 00:00:08