require 'date'

start_time =  "2012-03-02 14:46:21"
today_date =  Time.now.strftime("%Y-%d-%m %H:%M;%S")
end_time   =  "2017-04-02 14:46:21"

st, nt, et = [start_time, now, end_time].map do |t|
               # convert to unix timestamp, as float
               DateTime.parse(t).strftime('%s').to_f
             end
result = (nt - st) / (et - st) # assumes start < today < end