LAST MINUTE REMINDER: April 15 is tax day. Your federal income tax return must be postmarked not later than midnight Wednesday for the filing to be considered on time.

If for any reason you can't make the deadline, you can get an extension to June 15 by filing IRS Form 4868 by April 15. Approval is automatic.

This is a simple form, but it requires that you estimate your final tax liability and compare that with the amount of tax withheld or paid by quarterly estimate during the year.

If the estimated tax is less than the total paid or withheld, you can't claim a refund on Form 4868. That will have to wait until you file the complete return.

But if you expect to owe additional tax when you render the final accounting, then you render the final accounting, then you are required to attach payment to the 4868 for the amount of the estimated shortfall.

When you file your tax return later, remember to attach a copy of the completed Form 4868 to document the request for extension.

Maryland, Virginia and the District of Columbia also have provisions for requesting extensions. See the local instructions booklet for your home state.

Virginia residents may not need an extension. The Virginia state return isn't due until May 1, so if you expect to complete your work in the next couple of weeks, your Virginia return will be on time.

Question: I recently set up a "college fund" savings account for my child, on which the interest for 1980 amounted to $452. Is any of this taxable? If in the future the annual interest exceeds $1,000, will I still be allowed to claim the child as a dependent?

Answer: I can't quite decide whether to answer your first question yes or no, so I'd better explain the circumstances in detail.

Interest on a savings account is subject to tax -- that is, it's taxable income, as opposed to nontaxable income like municipal bond interest.

But if that's the only income your child had in 1980, he or she has no tax liability and in fact isn't even required to file a return. The filing minimum for one who is a dependent of another taxpayer is $1,000 of unearned income (such as taxable interest and dividends).

So when the annual interest hits $1,000, you will have to file a federal return for the child. If the personal exemption is raised in the future, you can expect the filing trigger to go up with it.

Even when the income reaches that level, you will probably be able to claim the child as your dependent, and the child will also get the personal exemption on his own return.

As long as the child is under the age of 19, or is a full-time student for at least five months of the year, the $1,000 income test for a dependent is waived.

The key requirement then is that you have contributed more than half of the child's living expenses during the year. And in adding up the child's own contributions, you include only the amounts actually spent by the child; interest that remains unspent in the account should not be counted.

Q: I enjoyed reading your tax guide [March 2] and found it very helpful. However, there is an error in the section on the energy tax credit. The restriction to principal homes completed homes completed before April 20, 1977, applies only to the conservation credit. The credit for renewable energy sources applies to both new and existing home.

A: You're right, and I thank you for pointing out the error and giving me a chance to correct it.

Q: Practically none of the money market funds publish their asset value on a daily basis. How can this information be obtained so that the investment can be properly monitored?

A: Virtually every money market fund maintains a fixed dollar value on all shares issued -- usually $1. Since their assets consist exclusively of money instruments that are carried at fact value, the underlying asset value per share doesn't change.

What does change is the yield, often on a daily or weekly basis. Recently some newspapers have begun quoting weekly yields for a number of money market funds.

You'll find a table of yield quotations right here in Washington Business every Monday. For this week's listing, look on page 16.