> # Time of dosing recorded in the dataset
>  
> datNew$*****S <-
Error: unexpected '^' in "datNew$**"
>  
>   with(datNew,
+  
+        ifelse(EVID == 1, TIME, NA))
[1]   NA    0   NA   NA   NA   NA   NA   NA   NA   NA   NA   NA   NA   NA  696   NA   NA   NA   NA   NA   NA   NA   NA   NA   NA 1416   NA   NA   NA   NA
[31]   NA   NA   NA   NA   NA   NA    0   NA   NA  696   NA   NA 1416   NA   NA    0   NA  696   NA 1416

when I change *****S to S, I got below dataset (datNew):, however, for the records with OBS = 11, 24, 35, TSLD should be 24 instead of current 0 in datNew. Thanks for further help in advance.

datNew = structure(list(ID = c(1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L,
2L, 2L, 3L, 3L, 3L, 3L, 3L, 3L), DAY = c(1L, 1L, 1L, 1L, 1L,
1L, 1L, 1L, 1L, 1L, 1L, 3L, 3L, 30L, 30L, 30L, 30L, 30L, 30L,
30L, 30L, 30L, 30L, 30L, 60L, 60L, 60L, 60L, 60L, 60L, 60L, 60L,
60L, 60L, 60L, 1L, 1L, 1L, 30L, 30L, 30L, 60L, 60L, 60L, 1L,
1L, 30L, 30L, 60L, 60L), TIME = c(0, 0, 1.16, 1.89, 2.88, 3.85,
4.95, 5.94, 8.01, 9.89, 24, 49, 49.5, 696, 696, 696.83, 698.01,
699.02, 700.11, 700.91, 702.06, 703.9, 705.93, 720, 1416, 1416,
1417, 1417.82, 1418.96, 1420.02, 1421.13, 1422.04, 1424.12, 1426.03,
1440, 0, 0, 2.09, 696, 696, 698.14, 1416, 1416, 1417.83, 0, 0,
696, 696, 1416, 1416), AMT = c(NA, 50000, NA, NA, NA, NA, NA,
NA, NA, NA, NA, NA, NA, NA, 50000, NA, NA, NA, NA, NA, NA, NA,
NA, NA, NA, 50000, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, 50000,
NA, NA, 50000, NA, NA, 50000, NA, NA, 50000, NA, 50000, NA, 50000
), MDV = c(0L, 1L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L,
0L, 1L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 1L, 0L, 0L, 0L,
0L, 0L, 0L, 0L, 0L, 0L, 0L, 1L, 0L, 0L, 1L, 0L, 0L, 1L, 0L, 0L,
1L, 0L, 1L, 0L, 1L), EVID = c(0L, 1L, 0L, 0L, 0L, 0L, 0L, 0L,
0L, 0L, 0L, 0L, 0L, 0L, 1L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L,
0L, 1L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 1L, 0L, 0L, 1L,
0L, 0L, 1L, 0L, 0L, 1L, 0L, 1L, 0L, 1L), II = c(NA, 24, NA, NA,
NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, 24, NA, NA, NA, NA, NA,
NA, NA, NA, NA, NA, 24, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA,
24, NA, NA, 24, NA, NA, 24, NA, NA, 24, NA, 24, NA, 24), SS = c(NA,
NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, 2, NA, NA,
NA, NA, NA, NA, NA, NA, NA, NA, 2, NA, NA, NA, NA, NA, NA, NA,
NA, NA, NA, NA, NA, NA, 2, NA, NA, 2, NA, NA, NA, NA, 2, NA,
2), ADDL = c(NA, 28, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA,
NA, NA, 29, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, 29, NA, NA,
NA, NA, NA, NA, NA, NA, NA, NA, 28, NA, NA, 29, NA, NA, 29, NA,
NA, 28, NA, 29, NA, 29), OBS = 1:50, S = c(NA, 0, NA, NA, NA,
NA, NA, NA, NA, NA, NA, NA, NA, NA, 696, NA, NA, NA, NA, NA,
NA, NA, NA, NA, NA, 1416, NA, NA, NA, NA, NA, NA, NA, NA, NA,
NA, 0, NA, NA, 696, NA, NA, 1416, NA, NA, 0, NA, 696, NA, 1416
), S.locf = c(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 696,
696, 696, 696, 696, 696, 696, 696, 696, 696, 696, 1416, 1416,
1416, 1416, 1416, 1416, 1416, 1416, 1416, 1416, 0, 0, 0, 0, 696,
696, 696, 1416, 1416, 0, 0, 0, 696, 696, 1416), II.locf = c(24,
24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
24), ADDL.locf = c(28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28,
28, 28, 28, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29,
29, 29, 29, 29, 29, 29, 29, 29, 28, 28, 28, 28, 29, 29, 29, 29,
29, 28, 28, 28, 29, 29, 29), TSLD = c(0, 0, 1.16, 1.89, 2.88,
3.85, 4.95, 5.94, 8.01, 9.89, 0, 1, 1.5, 24, 0, 0.830000000000041,
2.00999999999999, 3.01999999999998, 4.11000000000001, 4.90999999999997,
6.05999999999995, 7.89999999999998, 9.92999999999995, 0, 24,
0, 1, 1.81999999999994, 2.96000000000004, 4.01999999999998, 5.13000000000011,
6.03999999999996, 8.11999999999989, 10.03, 0, 0, 0, 2.09, 24,
0, 2.13999999999999, 24, 0, 1.82999999999993, 0, 0, 24, 0, 24,
0)), .Names = c("ID", "DAY", "TIME", "AMT", "MDV", "EVID", "II",
"SS", "ADDL", "OBS", "S", "S.locf", "II.locf", "ADDL.locf", "TSLD"
), row.names = c(NA, -50L), class = "data.frame")

I did not test your data but have recently dealt with a similar problem. Here I want to show you the method of how to do it.

1. First you need to extract the dosing event together with the unique ID of the data set, and create a new dataframe.

2. Merge the two data sets by the unique ID, which would give you a column of the dosing time.

3. What's left is pretty straightforward. You can simply do a elementary-school math to calculate the time after dosing.

Hope this helps.