President Reagan has signed legislation that, among other things, closes a longstanding retirement loophole for federal part timers and makes a payroll accounting change that will cost the average civil servant about $30 between now and Oct. 1.

Although the changes are only a minor part of the complex Consolidated Budget Reconciliation Act of 1985, they are of major interest to the 350,000 federal workers in the Washington area.

Despite its name, the bill cleared Congress only last month and was signed into law Monday evening.

Under the new law, part-time service performed after April 7 will be prorated for retirement purposes if the employe later moves into a full-time job prior to retirement.

Before the change, part timers could switch to full-time jobs for three years and then retire on the same pensions as career full timers.

The government has been basing pension benefits on years, but not hours, of service and the employes' highest average salary over three years.

The new law does not apply to part-time service before April 7.

But it will require such work after that date to be prorated on an hourly basis, rather than an annual basis, for retirement purposes.

The second change -- and this gets really complicated -- returns the government to an accounting system that computes salaries based on a 2,087-hour year, a reduction of seven hours annually.

The 2,080-hour year was standard in government for a long time. But several years ago Congress as a temporary measure told agencies to compute pay on a 2,087-hour year.

That meant a slight drop in take-home pay for government employes, but saved the government millions of dollars each year.

The temporary change expired before Congress, which has been working on the reconciliation bill for more than a year, could extend it.

The result was that some agencies switched back to the 2,080-hour computation while others continued to use 2,087 hours.

Now, with the new law, agencies have been told to base all their payrolls on 2,087 hours.

It won't require anybody to work longer, but it will mean a slight dip in pay -- on average, about 20 cents a week -- for many employes as their hourly rates are adjusted.

It also will mean nightmares for payroll offices that must readjust to the old system. IRS Checks

Many of the 2,554 Internal Revenue Service workers in the District, Maryland, Virginia and West Virginia whose paychecks are sent to their homes didn't get them Monday.

IRS officials say that the checks were picked up by the U.S. Postal Service from the Washington Disbursing Center, but were not delivered.

IRS says that special arrangements will be made to get supplemental checks to employes.

Meanwhile, lots of people are looking for several missing mailbags.

This does not affect employes whose checks are sent directly to banks, or to those workers who still get their checks at the office. NBS Old Timers

The National Bureau of Standards Alumni Association will hold its first meeting tomorrow at the NBS in Gaithersburg. For details call Reeves Tilley, 762-7186.