The Trump administration announced Friday that it will tighten the rules on enforcing visa overstays for foreign students and exchange visitors as part of an effort to further crack down on unlawful immigration.

U.S. Citizenship and Immigration Services (USCIS) said that the agency will change the policy for calculating the length of certain visa overstays, which previously was counted in days starting from when the violation was first reported.

Under the new policy, USCIS will instead count each day since a visa holder's status has lapsed in order to determine how long someone has been "unlawfully present" in the U.S.

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The change is significant because length of visa overstays determines whether an immigrant is eligible to reenter the United States in the future.

According to the USCIS, overstays of more than 180 days can lead to 3- or 10-year admission bans, while overstays of longer than a year lead to permanent bans.

USCIS Director L. Francis Cissna said the new policy was meant to send a clear message to immigrants who overstay their visas.

The Department of Homeland Security estimated that 629,000 people overstayed their visas in 2016, the most recent year on record.

“USCIS is dedicated to our mission of ensuring the integrity of the immigration system. F, J, and M nonimmigrants are admitted to the United States for a specific purpose, and when that purpose has ended, we expect them to depart, or to obtain another, lawful immigration status,” Cissna said in a statement.

“The message is clear: These nonimmigrants cannot overstay their periods of admission or violate the terms of admission and stay illegally in the U.S. anymore.”

There is a 30-day comment period before USCIS will implement the rules, and will go into effect on Aug. 9.