.

In[1]:= raw = Import["2015-06-25_TemperatureValues.xls"];
data = Map[Flatten[{AbsoluteTime[#[[1]]], Rest[#]}] &, Drop[First[raw], 7]]

Out[2]= {{-86399.96300000000000000, 32.17, 34.96},
 {-86399.92600000000000000, 32.17, 34.96},
 {-86399.8890000000000000, 32.17, 34.96},
 {-86399.8520000000000000, 32.17, 34.96},
 {-86399.8150000000000000, 32.17, 34.96},
 {-86399.7780000000000000, 32.17, 34.96},
 {-86399.7410000000000000, 32.17, 34.96},
 {-86399.7040000000000000, 32.17, 34.96},
 {-86399.6670000000000000, 32.17, 34.96},
 {-86399.6300000000000000, 32.17, 34.96},
 {-86399.5930000000000000, 32.17, 34.96},
 {-86399.5560000000000000, 32.17, 34.96}}

In[3]:= g1 = Show[ListPlot[Map[{#[[1]], #[[2]]} &, data]],
  ListPlot[Map[{#[[1]], #[[3]]} &, data]]]

In[4]:= raw = Import["testdata.txt"];
data = Map[Flatten[{AbsoluteTime[#[[1]]], ToExpression[Rest[#]]}] &, 
  Map[StringSplit[#, "\t"] &, StringReplace[Rest[StringSplit[raw, "\n"]], "," -> "."]]]

Out[5]= {{3629059200, 37, 32.17, 34.96},
 {3629059200, 74, 32.17, 34.96},
 {3629059200, 111, 32.17, 34.96},
 {3629059200, 148, 32.17, 34.96},
 {3629059200, 37, 32.17, 34.96},
 {3629059200, 74, 32.17, 34.96},
 {3629059200, 111, 32.17, 34.96},
 {3629059200, 148, 32.17, 34.96},
 {3629059200, 185, 32.17, 34.96},
 {3629059200, 222, 32.17, 34.96},
 {3629059200, 259, 32.17, 34.96},
 {3629059202, 333, 32.17, 34.96},
 {3629059203, 370, 32.17, 34.96}}

In[6]:= g2 = Show[ListPlot[Map[{#[[1]], #[[3]]} &, data]],
  ListPlot[Map[{#[[1]], #[[4]]} &, data]]]

Your spreadsheet has December 31 year 1899 with times over 1/5 of one second.

Your text file has no month or day or year with times over 1/9 of one second.

If your month day year and time were so useful number then graph is better.

Or subtract date and time of first row from all other rows and get graph that starts at zero.