The respective rounds of the referendum will be divided into two regions, it said. The draft constitution, approved by an Islamist-dominated council last month, has become the focus of Egypt's worst political crisis since President Mohamed Morsi's election in June.
Rival mass rallies have become an almost daily occurrence in Cairo, and clashes between Islamists and secular-leaning opposition last Wednesday killed seven people and injured hundreds more.
The army has called for talks later on Wednesday between Morsi and opposition leaders.
Troops and tanks were deployed outside the presidential palace on Tuesday to prevent further bloodshed, but exercised restraint when protesters partly destroyed roadblocks to get closer to the complex and demand Morsi's departure.
The expatriate vote has already begun with the polling of more than 500,000 Egyptians at embassies and consulates.