request.localnow = request.now # creation of variable localnow
if not session.timeshift: # cretion of variable timeshift and save it to session
    session.timeshift = request.localnow - request.utcnow 

This model must be executed before any call of the variables session.timeshift or request.localnow. I personally create models/0_.py for this. 0_.py always executes before other models.

• Create file modules/helpers.py and put there the following code

  from gluon import current
  
  # utc : datetime
  def utc2local(utc):
      return utc+current.session.timeshift
  
  # utc : datetime
  def local2utc(local):
      return local-current.session.timeshift

• Everytime that you save a datetime to database save it as UTC datetime. Use local2utc() function for the conversion.
• In every view use utc2local() function to convert UTC datetime from the database to the local timezone of each user.