What is the filing date for applications properly filed under 37 CFR 1.10 (Priority Mail Express)?

For applications properly filed under 37 CFR 1.10 using Priority Mail Express, the filing date is the date the application was deposited with the U.S. Postal Service. The MPEP states:

For applications properly filed under 37 CFR 1.10, the filing date is the date that the application was deposited as Priority Mail Express® in the U.S. Postal Service. For example, if a new patent application is deposited in Priority Mail Express® in accordance with 37 CFR 1.10 on a Saturday and the United States Postal Service gives it a date of deposit of Saturday, the Office will accord and stamp the correspondence with the Saturday date.

If the proper procedures were not followed, the application will receive a filing date as of the date it was received in the USPTO.