I am planning to travel to Cancun, Mexico. I am a Ukrainian citizen with a U.S. student visa. I am having great difficulty submitting this form, as it will not allow me to enter the date of arrival. I have tried calling Mexican consulates with no luck.
According to this page: http://visasmex.com/files/SAE_eng.pdf an electronic authorisation is valid for 30 days from date of issue. Hence it seems you cannot apply for this authorisation more than 30 days in advance.
Since you have a valid US visa, you do not need a Mexican visa or electronic authorization to visit Mexico.
Starting May 1st, 2010, all foreigners, regardless of their nationality, visiting Mexico as tourists, transmigrate or for business matters will not need a visa for Mexico as long as they hold a valid US visa and arrive in Mexico from the United States.