Unfortunately, the editorial "Darkness at 12 p.m. -- or Is It a.m.?" {Dec. 15} has not been as helpful as it could have been in the matter of educating the public on how to avoid potentially costly litigation. It is too bad that over the desire to write something interesting, the substance has been lost. But at least you agree with me that there is an ambiguity problem.

The information sheet that we send out gives three options, not just the one you mention, on how to avoid the ambiguity. After reading it, you will agree that it is a very simple matter for people to be specific in one of the recommended ways. For computers (and for people who have absorbed some of the spirit of mathematics), the best solution is what the scientific and technical world has done for a long time: use the 24-hour system consistently. In this country only the military saw the problem clearly and adopted the system. That is why this system is now identified with the military.

You seem to imply that because it is "military," we can't use the 24-hour system. But you should know that nowhere else in the civilized world do people have this hang-up. It is perfectly acceptable for a radio announcer in Italy to say the equivalent of "It is 20 hours exactly" at the beginning of the evening news, or for people to agree on times such as "1930 at San Marco."

To sum up: it is important to make people aware that they may be misunderstood if they use a.m. or p.m. for the designation of noon or midnight. This would be a public service. Our sheet is intended to assist them with options for unambiguous alternatives. But beyond that, we should strongly urge the public to drop the use of the antiquated and really obsolete a.m./p.m. system of telling time. Our modern world requires increasing precision of expression, and that may be one of the reasons why this controversy is increasing also. -- Gernot M. R. Winkler The writer is director of the Time Service Department at the U.S. Naval Observatory. The Information Sheet A question frequently asked is whether 12 a.m. and 12 p.m. should be used to denote noon and midnight, respectively, or vice versa. The answer is that these abbreviations should not be used. They will cause confusion.

The abbreviation 12m is sometimes used to denote noon (m denotes meridian). However, 12m is likely to be interpreted as an abbreviation for midnight. Hence, 12m should not be used either.

The following are three solutions to the problem of designating noon and midnight unambiguously:

1. Use the complete words "noon" and "midnight." If midnight is used, give the two dates between which it falls. Thus, for example, "midnight of 21 September" is ambiguous but "midnight of 21/22 September" is specific.

2. Prepare schedules with times other than noon and midnight. Use times such as 12:01 a.m., 11:59 p.m., etc. This is done by railroads.

3. Use the 2400 system, which is used by international airlines and the military services. The first two digits give the hours past midnight and the second two give the minutes. Noon is designated by 1200. Midnight is designated as 0000 of the new date, i.e., midnight of 21/22 September is Oh,22 September.