@txnelson is on the right track. It seems that there is a problem with the syntax you are trying to use to reference the columns. In the first line of code you shared, you have
:"Date Info"
The column is created, but it has no values. Try the following and see if it works.
dt << New Column( "AM or PM", character, Formula( Word( 5, :Date Info ) ) );
dt << Color or Mark by Column( :AM or PM, Marker Theme( "hollow" ) );
Wendy