Applying dates to files

[pplob 0]

Beginner here. I am looking for guidance on visualizing change over time among documents. I have a series of documents over a 30-year period, and I'm trying trace changes in the proportion of different codes over time. I need to be able to apply dates to files, and then summarize those proportions for different years.

Any guidance on how to do this? Elsewhere I've seen people using "attributes" for this purpose but I can't figure out how to do that consistently. Instructions or pointing toward existing documents/trainings on this would be helpful. I'm using NVIVO 1.2.