The string format for your dates includes "-" and " " and ":". The documentation for Datelist claims you are supposed to be able to give it a format string and get it to extract a date from a string, but repeated attempts to get that to work have failed. Carefully study the details of the documentation and see if you can give it a string format that works. OR just do a little string hacking and extract your own fields and be done with it.
In[1]:= data = Import["data.tsv"];
dates = data[[;; , 1]];
fixeddates = Map[ToExpression["{" <> StringReplace[#,
{"-" -> ",", " " -> ",", ":" -> ","}] <> "}"] &, dates]
Out[3]= {{2014, 3, 29, 8, 39, 37.0808}, {2014, 3, 29, 8, 39, 39.4492},
{2014, 3, 29, 8, 39, 42.6031}, {2014, 3, 29, 8, 39, 44.8829},
{2014, 3, 29, 8, 39, 47.022}, {2014, 3, 29, 8, 39, 48.9519}}
where each date is now a list of numbers.
Ah, a little progress on getting formatting strings to work. This works
In[18]:= DateList[{"2014-03-29 08:39:48", {"Year", "-", "Month",
"-", "Day", " ", "Hour", ":", "Minute", ":", "Second"}}]
Out[18]= {2014, 3, 29, 8, 39, 48.}
but this doesn't
In[19]:= DateList[{"2014-03-29 08:39:48.951891", {"Year", "-", "Month",
"-", "Day", " ", "Hour", ":", "Minute", ":", "Second"}}]
During evaluation of In[19]:= DateString::str: String 2014-03-29 08:39:48.951891 cannot be interpreted as a date in format {Year,-,Month,-,Day, ,Hour,:,Minute,:,Second}. >>
Out[19]= DateList[{"2014-03-29 08:39:48.951891", {"Year", "-",
"Month", "-", "Day", " ", "Hour", ":", "Minute", ":", "Second"}}]
So Datelist will extract Integer seconds but not Real seconds even though the documentation says "Day, hour, minute, and second values can be nonintegers:"
Unless someone else can come up with a good explanation that really looks like a bug.
Edit showing how to then plot the data
data = Import["data.tsv"];
points = data[[All, {1, 3}]]; (*extract date and 3rd column*)
fixedpoints = Map[{ToExpression["{" <> StringReplace[First[#],
{"-" -> ",", " " -> ",", ":" -> ","}] <> "}"], Last[#]} &, points];
DateListPlot[fixedpoints, PlotStyle -> PointSize[0.02]]
...PlotSnipped...