A CALENDAR DATE CALCULATOR

by

Peter W. Voisey

Engineering Research Service

Research Branch, Canada Department of Agriculture, Ottawa

The need to determine the numberof days between given calendar datesor vice versa frequently arises inagricultural research and production.The elapsed time before certainactions must be taken in the reproductive and productive cycles of animals and plants must be controlledand scheduled. For example, gestation in animals, movement of hensto laying batteries and the plantingor harvesting of crops.

Time is expressed as calendar datesbecause these relative points in timeare universally used and understood.Conversion of time periods to datescan be made by a computer but formany purposes this is not convenient.Counting days manually on a calendar is time consuming and subject toerror particularly if numerous conversions must be made. A solution isthe circular calculator of which thereare many special types manufactured.However to the author's knowledge aconversion calculator for calendardates has not been marketed, although nomograms and tables havebeen published by farm suppliers forthis purpose in advertising literature.

A circular date calculator was described by Abel (1). A disc withits perimeter divided into 182 equalparts to form the inner scale wasrotated above a second disc whoseperimeter was similarly divided butmarked in the order of calendar daysand months to form the outer scale.This instrument had the disadvantages that it could not be used forperiods greater than 364 days andif a period passed through Decemberinto January one day must be addedor subtracted from the result to makethe conversion. Also each division onboth scales represented two days andthe inner scale was marked off ineven days and the outer in odd oreven days depending on the numberof days in the month. Thus this calculator was somewhat confusing par-

1. Contribution No. 133 from the Engineering Research Service.

42

ticularly for unskilled labour. Thereason the author used this arrangement was probably the practicalproblem of dividing a circle into 365equal parts.

To be useful any special calculatorshould be easily reproduced to spreadthe cost of preparing the scales andto make copies readily available at amodest cost. The techniques used toproduce a specific gravity calculatorfor potatoes (3, 4) were thereforeused to produce a calendar date calculator.

The circumference of a 38 cmdiameter circle engraved on an aluminum sheet was precisely divided into365 parts using a milling machineequipped with a dividing head anddifferential attachment (2). Thismaster scale was traced on to mylarsheets. The scale on one sheet wasmarked with days and months to produce the outer scale and the graduations on the other sheet were markedconsecutively 1 to 365 and 365 to 730to produce the inner scale. In addition the operating instructions wereprinted in the mid-area of the innerscale. The two mylar sub-masterscales were then reduced to one-thirdof size and printed photographicallyon 0.51 mm thick vinyl sheet. Thescales were then trimmed to size andassembled by a plastic pivot in thecenter (3).

The resulting calculator ( Fig. 1) ispocket size (12.7 cm diameter) andhas a yellow legend printed on ablack background. This colour combination provides high legibility forsuch high density scales.

To determine the number of davsbetween known dates, zero on theinner scale indicated by a pointer, isset opposite the first date on th''outer scale. The interval is thennoted on the inner scale opposite thesecond date. To determine the datesfor a given number of days the scalesare read in the opposite direction.

The calculator can be read directly

for intervals up to two years. Iflonger periods are required the corresponding multiple of 365 must beadded to the conversion. Leap yearsare not accounted for and one daymust be added to the conversionwhenever the end of February ispassed for a calculation in a leapyear. This could be readily overcomeby constructing a special leap yearmodel with 366 divisions.

Arrangements have been made toproduce the calculator commercially.It is estimated that they may costabout $4 (5).

REFERENCES

1. Abel, G. H. 1967. A rapid calendardate counter. Agron. J. 59: 382-383.

2. Oberg, E. and Jones, F. D. 1959.Machineries Handbook. The Industrial Press, p. 1305.

3. Voisey, P. W., Hansen, II. andThomlison, A. W. 1964. A specificgravity calculator for potatoes.Eng. Bull. 6253. Eng. Res. Service,Canada Dept. Agr., Ottawa.

4. Young, D. A., Voisey, P. W., andNixon, N. 1964. A specific gravitycalculator for potatoes. Amer.Potato J. 41: 401-405.

5. Private Comm. INFO Inc., Newton, Mass. 02158, U.S.A.

Figure 1: The date calculator.

CANADIAN AGRICULTURAL ENGINEERING, VOL. 10, No. 1, MAY 1968