First-to-file patent systems such as the requirements followed in the European Union place more importance on the filing date than the actual date when the invention was conceived. In the US, the PTO determines the priority between two applications by considering the invention date as well as the filing date. Thus, the USPTO can be considered to follow a "first-to-invent" doctrine. An inventor can publicly disclosure the invention (i.e. demonstrating that the invention has been fully conceived) and wait to file a patent for up to a year in the US. This is not the case in Europe since any public disclosure is considered to be prior art. Consequently, only the filing date matters. Article 60(2) of the European Patent convention (EPC) awards priority to the applicant that first files the invention. Under first-to-file patent systems inventors are more pressured to prepare and file the application as soon as possible in order to gain the IP protection for their invention.

References:
[1] Stim, R. "Intellectual Property. Patents, Trademarks, and Copyrights" West Legal Studies.
[2] Black's Law Dictionary 5th ed., (West Publishing, 1979).
[3] Manual of Patent Examining Procedure, 8th Edition
[4] The European Patent Convention