Tax Calendar: Today is the due date for payment of the second quarterly installment of estimated tax for the federal government and also for the District, Maryland and Virginia. Today is also the last day for filing 1978 income tax returns for which an automatic two-month extension had been requested earlier (on Form 4868).

Savings Bond Update: The Treasury Department has announced an increase in the interest rate for U.S. savings bonds from 6 percent to 6.5 percent, effective June 1.

The new rate applies to Series E. bonds bought after June 1, 1979, if they are held until their initial maturity dates. Bonds redeemed earlier will yield something less than the full 6.5 percent.

The increase also applies to older E. bonds, effective with the first semiannual interest period beginning after June 1. (Like new purchases, bonds bought in recent years must be held at least to the original maturity date to qualify for the 6.5 percent rate.)

The term for all E bonds remains five years; the extra interest will be paid as a one-time bonus at maturity.

Series H bonds bought after June 1 also will earn 6.5 percent if held to maturity. And semiannual interest checks on presently owned H bonds will go up starting with the first semiannual interest period beginning after June 1.

The improved rate of return will apply to the new Series EE and HH bonds which go on sale Jan. 2, 1980. Semiannual checks for HH bonds will reflect the 6.5 percent rates, but there will be an adjustment for HH bonds redeemed less than five years after purchase (unless they were obtained in exchange for E or EE bonds).

The 6.5 percent return on the EE bonds will be obtained by shortening the term of original maturity to 11 years from the 11 years, 9 months first announced

Question: My husband and I filed a joint tax return for 1978, but our tax prepare said we could only take a $100 dividend exclusion. I thought there is a $200 exclusion on a joint return.

Answer: The Basic exclusion is $100 per taxpayer; this may be as much as $200 on a joint return. But you can claim the double exclusion only if the dividend-paying stock is owned jointly, or if you and your husband each owned stock which generated $100 in qualifying dividends.

Apparently all of the dividends qualifying for the exclusion, you will have to place enough stock in joint ownership to generate $200 a year in dividends, or be sure that each of you earns $100 in qualifying dividends each year on separately owned stock.

Q: I've been operating a consulting business from an office in my home for a number of years. We've now considering selling our house to take advantage of the new $100,000 tax exclusion. Are there any complications?

A: Yes - if you have claimed deductions in the past for the office in your home. You may only exclude gain on the sale of that part of the house used as a personal residence. You will have to report and pay tax on the gain attributable to the part used as business property.

You have some figuring to do - and the calculations can get pretty complicated, because you have to recover any depreciation you have claimed as a business expense over the years. It gets particularly messy if you had used an accelerated depreciation method - that is, anything faster than straight-line depreciation.

You may want to get some professional help on this one. Take a look at IRS Publications 523 (on sale of a residence) and 544 (sale of business property) for detailed information.