The date from which patent right is deemed to start is usually the date of filing of complete specification. To obtain rights in other member countries, the application must be filed on the same day in other member countries if it is desired to have the rights started from the same day.
However, there are practical difficulties in synchronizing the activities. For facilitating simultaneous protection in member countries, the Convention provides that within 12 months of national filing, if patent applications are filed in those member countries, the patents, if granted in member countries, will be effective from the date of national filing. This right is known as the right of priority.
In other words you maintain the priority or the same date of filing in all the member countries and no one else in those countries can obtain the patent rights on a similar/identical invention from the same or a later date.
In case the applicant after a second look at the patent application finds that the patent contains more than one invention or on his own accord wishes to divide the application, he can claim the initial date of priority for subsequent patent applications.
The applicant may also, on his own initiative, divide a patent application and preserve as the date of each divisional application the date of the initial application and the benefit of the right of priority, if any. Each country of the Union shall have the right to determine the conditions under which such division shall be authorized.
Priority may not be refused on the ground that certain elements of the invention for which priority is claimed do not appear among the claims formulated in the application in the country of origin, provided that the application documents as a whole specifically disclose such elements.