Scores of worried taxpayers who called or showed up at the Arlington County Courthouse yesterday to inquire about the Sept. 15 expiration date printed on their current automobile license tag decals found that they have another month before they have to display new decals.

While yesterday was the deadline for paying personal property taxes in the county without penalty, county Treasurer Frank O'Leary said that under rules approved by the Arlington County Board last spring, new tags for the county's 114,000 registered vehicles do not have to be displayed until Oct. 15.

O'Leary, wearing a button that read "Don't Panic," said that until this year's rules change, Sept. 15 had been the annual expiration date and was printed on tags issued last year.

For the first time, Arlington residents have been allowed this year to pay their taxes using major credit cards. As many as 6,000 are expected to do so by next week, according to O'Leary.

The credit card program, meant to help people without cash pay their taxes on time, has been very popular. "Apparently there are a lot of people in Arlington who like the idea and find it very convenient," he said.

For the last several years, the date for the decals to be displayed on license tags, issued through the mail after payment of property taxes, has been extended to October.

The extensions were largely because of computer breakdowns and widespread postal delays, according to O'Leary. He said there have been fewer problems this year and an extension beyond Oct. 15 is unlikely. After the expiration date, vehicles without the required sticker can be given $25 tickets each day until the sticker is displayed.