My parents have got Schengen visas for 30 days. The flight starts from India on July 11th and reaches Zurich on July 12th. Their return ticket is for August 10th. They will stay exactly 30 days in the Schengen area. Is visa validity counted from the day one leaves his country or from the day he enters the Schengen region?
The date that matters is the date on the stamp they will receive when going through the passport control in Zurich so the date at which they enter the Schengen area. Both the day they arrive and the day they leave count so on August 10 your parents will have stayed exactly 30 days and will be able to leave without violating the conditions of their visa.