This might be too late for you. I had the same question and was able to find the descriptions and wanted to post answer just in case someone else searches for this in the future.

https://tcga-data.nci.nih.gov/docs/dictionary/

To update the old answers, the data dictionary has once again been moved.

https://docs.gdc.cancer.gov/Data_Dictionary/viewer/

As for the "days_to_drug_therapy_start" and "days_to_drug_therapy_end" columns of the drug annotation file, the little information I could find suggests these to be days from the date of initial pathological diagnosis.

https://docs.gdc.cancer.gov/Data_Dictionary/viewer/#?view=table-definition-view&id=treatment (lists days_to_treatment_start as "Number of days between the date used for index and the date the treatment started"; does not specify what the index date is)

http://docs.cavatica.org/docs/tcga-grch38-metadata#section-treatment (defines "days to treatment" as "The number of days from the date of the initial pathologic diagnosis that treatment began.").

Hope this helps others having to work with TCGA drug and radiotherapy annotation files.