Subtract times over 48 hours

I have been given good ideas and suggestions , but I cannot get this to work. I do not want to have to enter the date as well as the time in each cell. This is too labor intensive. I like the idea of using +1 , +2 e.t.c as the time passes from one day to the next. I show two subjects receiving a drug. The time of dosing in the evening (21:05) is shown in cell B2 and this time has to be subtracted from each of the the time intervals shown 0, 4, 8...............

I cannot get this to work across 48 hours. I enclose an example in Excel 2007. I just cannot see what I am doing wrong. Could I be advised.

Hello! Thank you for your contribution. This is exactly what I am looking for. I am looking at the data structure at the moment to see if I understand it well. I am asking my self why is this working?

Angus

Willem: I have found out that it works for some of the blood sampling times tims, but not all of them. The problem begins at row 12 when the time of blood sampling at 0 time is prior to time of drug administration: this is quite common in clinical studies. I enclose the data set of times. Comments are welcome to resolve this problem. I do not understand the routine well.

Willem: many thanks .......there has to be a conditional statement. I do not know the Excel language. I will try it with the full data set later today to see if there are other problems to be addressed.

Angus

I have got it to work now. But I have a question. If you look at the worksheet you sent to me as an example I find that something odd has happened to cell C9. it used to show -0.08. It has changed and I do not understand why.