You could fiddle with the param in the controller. Here is a mock-up:
now = Time.now
param_date = params[:project_time][:date]
params[:project_time][:date] = case
when /^\d$/ then Time.mktime(now.year, now.month, param_date)
...
else param_date
end
I found another solution, which I think is the way to go, as the date
is corrected just as it is added to the model.
So the model always holds correct data.
I overwrite the date= method in my model
class ProjectTime < ActiveRecord::Base
...
def date=(value)
  # check value and do the filtering
  value = ....
  # set attribute im my model class
  self[:date] = value
end
