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Calendar Time

Stephen Greb and Brandon Nutall, Kentucky Geological Survey

Calculating/converting geologic ages onto a calendar year

Calculating from the beginning of the year

(6a.1) To calculate from beginning of the year:

365 – (Date or age of creature in million years / 12.603 years/day) = Decimal days from beginning of year

For example, T-rex lived 75 million years ago, so:

365 – (75 my / 12.603 yr/day) = 365 - 5.95 days = 359.05 day of the year

(6a.2) To get the month and date of the calendar year from your calculated decimal days from the beginning of the year you can use the following chart. Use the whole number in the decimal date you calculated to determine month and day.

This chart may be helpful for determine what date the decimal day is:

Month Days from beginning year
January 1, just past midnight 0
February 1 (0+31) 31
March 1 (31 +28) 59
April 1 (59+31) 90
May 1 (90+30) 120
June 1 (120+31) 151
July 1 (151+30) 181
August 1 (181+31) 212
September 1 (212+31) 243
October 1 (243+30) 273
November 1 (273+31) 304
December 1 (304+30) 334
December 31 st midnight 365

In our example, if we look up 359.05 days on the chart we can see that it is in December between December 1st and 31 st .

359.05 – 334 = 25.05 after Dec 1, so is 25.05 + 1 =26.05 or Dec. 26 th (plus some remainder)

(6a.3) Next, to determine the time on that day in hours:

Decimal remainder of X * 24hrs/day = Y hours

In our example, 359. 05 days was calculated from equation 6a.1, and 26.05 was determined to be the date using the chart in step 6a.2. The decimal remainder is .05 from the beginning of the day so:

.05 days * 24 hrs/day = 1.2 hours from the beginning of the day, or 1 am (+ 0.2 remainder of a day)

(6a.4) Next, to determine the minutes of the day:

Decimal remainder of Y * 60 mins/hour = Z minutes

In our examples we calculated 1. 2 hours in equation 6a.3 and the decimal remainder was 0.2, so:

0.2 * 60 = 12 minutes past the hour or 1:12 am

(6a.5) Combine 6a through 6a.4 to get the position on the calendar:

X days from the end of the year, Y hour: Z minutes

For our example, Dec. 26th, 1:12 am

(6a.5) If you wanted more detail you could calculate to seconds by taking the remainder from step 6a.4 (if there is one) and multiply by 60 seconds.


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