Ok, one last beginner question (I wish there was a basic questions forum). I applied the changes above (thank you) and the data looks correct (I replaced the missing or bad data with "Missing[]" as online guidance suggests). But when I try and plot the data (using DateListPlot) with the date object on the x-axis and one of the data columns (say column 3 (sysBP) as y-axis values it doesn't work. I tried to transpose the data to create a {date, value} list and that works for all numerical data - but doesn't seem to work when one column is a date object. This seems so basic, import data with a date stamp and plot against another column of readings. I have read the online guidance but its pretty thin for date object import and plotting. I appreciate any advice on this basic ask.

Roger: A bit late but hope the following helps.

SetDirectory[$UserDocumentsDirectory];
data = Transpose[
   Delete[Transpose
     [ReplaceAll[
      Import["BP_readings_mathematica_test.xlsx", 
       {"Data", 2},
        HeaderLines -> 1, 
       "EmptyField" -> Missing[]],
      {"  --" -> Missing[], 
       " -" -> Missing[], {Missing[], Missing[], Missing[], Missing[],
          Missing[], Missing[], Missing[]} -> Nothing}]] ,
    3]
   ];
bprange[s_, d_] := Which[
   And[LessEqual[s, 120], LessEqual[d, 80]], 
   RGBColor[0.65, 0.81, 0.22],
   And[Between[s, {121, 129}], LessEqual[d, 80]], RGBColor[1, 0.93, 0],
   Or[Between[s, {130, 139}], Between[d, {81 , 89}]], 
   RGBColor[1, 0.71, 0],
   Or[Between[s, {140, 179}], Between[d, {90 , 119}]], 
   RGBColor[0.73, 0.23, 0.01],
   Or[GreaterEqual[s, 180], GreaterEqual[d, 120]], 
   RGBColor[0.6, 0.03, 0.07]
   ];
createBPseries[{date_, time_, systolic_, diastolic_, pulse_, 
    comment_}] := {
   Tooltip[
    Style[
     {
      DateObject[
       Join[DateValue[date, {"Year", "Month", "Day"}],
        DateValue[time, {"Hour", "Minute", "Second"}]
        ]
       ],
      Interval[{systolic, diastolic}]
      }, bprange[systolic, diastolic]],
    comment]};
{women1839, men1839, women4059, men4059, women60, 
   men60} = {{110, 68}, {119, 70}, {122, 74}, {124, 77}, {139, 
    68}, {133, 69}};
fixeddata = createBPseries /@ data;
DateListPlot[Flatten[fixeddata], 
 Joined -> False,
  PlotRange -> 
  MinMax[Flatten[Transpose[{data[[All, 3]], data[[All, 4]]}]]] + {-5, 
    5}, 
 AspectRatio -> 1/GoldenRatio,
 IntervalMarkers -> "Bars", IntervalMarkersStyle -> Thick,
 GridLines -> {None, men4059}
 ]

Changing {"Data", 2}, to {"Data", 1}, gets the first sheet instead of he second sheet of your spreadsheet.

Is NormalizeDateTime a built in function?

No. I showed how I defined it.

I was told to use lower case for user defined functions

Yeah, this is common advice, but it's silly. The system will alert you if you've tried to override a built-in function, so there's no real danger, and I like maintaining the uniform capitalization.

How did it know to join the first two columns with the date join command?

Because I used pattern matching. It's too much to go into as part of this answer. It sounds like you should run through some of the tutorials for how to program in Mathematica.

And is it easy enough to change GMT to EST with a command?

Look into TimeZoneConvert.