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Earth Science Education
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.Continue to calculated dates for Calendar Time
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