Here is the way OpenERP handles data that are location specific:

• Data are stored in the database in UTC (not depending on the timezone of your users)
• When the client receives the data, it translates it according to the timezone of the user which is on the global context that comes from the user preferences (so, the date/time you see on the screen is not the same than the one in the database)
• The client sends back the new dates to the server under the UTC format

For information, the fields that are in the global context come from the context_get() method on the res.users object. You may have to overwrite this method if you want to add a new field in the global context which is sent to the client when the user logs in .

So, I think you should develop a module that:

• _inherit the res.users object to add a preference field.selection whith the calendar he uses (gregorian / Solar Hijri)
• overwrite the default calendar widget in the web client to use a .JS widget that support Solar Hijri calendar
• do the translations between Solar Hijri and UTC when reading/writing dates in the widget (the current widget alreaady does this for all UTC timezones.)