Not sure what you mean by "adds days" and also without seeing your code (post only the relevant lines, not 500 lines if irrelevant code) it is not easy to see what you are trying do do when you "count trading days".
The line takes an absolute date as a parameter (not "trading days" or something like that) and if you specify the exact date of next Tuesday then it will plot the line at exactly this date. You can either use the date and time functions to calculate the timestamp for next Tuesday by converting a string to a timestamp or you can simply add the number of seconds to some existing timestamp to arrive at some date in the future. To plot a line exactly one week from now you would calculate
TimeCurrent() + 7 * 24 * 60 * 60
I have recently developed a custom indicator that counts trading days and plots vertical lines at my prescribed intervals. Works fine within the processed bars, however, as soon as it starts plotting in the future - to the right of the current bar - it adds days?? The code that I use is simply a loop that counts the number of days that aren't Saturday or Sunday, so I am quite sure that there is not a problem with the code. That suggests that the future bars might be referenced in a different manner than those already processed....
So far, it seems that it adds either 4 or six days to the count so I am guessing that it is the future weekends causing my problem. Has anyone else encountered this issue?