I am working on a Gantt Chart for projects, and need to add different milestones in the calender bar.

I am following the milestone guide from David McLachlan **(**https://www.youtube.com/watch?v=tuQpGzL_14g). But given that I need more than one milestone, I have tried to add the OR function – without luck as it only gives an error code.

This is the code that is not working:

=OR(IF(P$16=($F18-WEEKDAY($F18;2)+1);"u";""); IF(P$16=($G18-WEEKDAY($G18;2)+1);"S";"");IF(P$16=($H18-WEEKDAY($H18;2)+1);"z";"");IF(P$16=($I18-WEEKDAY($I18;2)+1);"v";"")) 

What am I doing wrong?