Time after dose

Hi, Everyone: I am having trouble with getting the correct output of TAD (time after dose) and hoping to get your suggestions. Here is a brief description of my dataset: drug was given multiple dosing every 12 hours. All the dosing events were recorded in the input file. Observations were taken right before the last dose (predose), and 0.5, 1, 2, 4, 8 hrs post-dose. I have used this code to calculate the TAD IF(AMT.GT.0) THEN TDOS=TIME TAD=0.0 ENDIF IF (AMT.EQ.0) TAD=TIME-TDOS In the output, the TADs calculated for the predose observation, instead of being a negative value, are closed to 12 and larger than the one for 8 hrs observations. It is obviously wrong and it give me wrong individual plots. I tried to find useful information in NM archive but cannot find any that suitable for my situation. Any suggestions are greatly appreciated! Thank you in advance for your help! Siwei