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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 end of the year
(6b.1) To back-calculate from the end of the year:Date or age of creature in million years / 12.603 years/day = Decimal days from the end of the year
For example, T-rex lived 75 million years ago, so:
75 my / 12.603 yr/day) = 5.95 days from the end of the year
(6b.2) To get the month and date of the calendar year from your calculated decimal days from the end of the year you can count back from New Year's if the number is small or use the following chart.
This chart may be helpful for determine what date the decimal day is:
| Month | Days before end of year |
| January 1, just past midnight | 365 |
| February 1 | 334 |
| March 1 | 306 |
| April 1 | 274 |
| May 1 | 244 |
| June 1 | 213 |
| July 1 | 183 |
| August 1 | 152 |
| September 1 | 121 |
| October 1 | 91 |
| November 1 | 60 |
| December 1 | 30 |
| December 31 st midnight | 0 |
In our example, 5.95 days, so count backwards from New Years Eve (Dec. 31) 5 days and part of 1 day, which would be Dec. 26 th (plus some remainder)
(6b.3) Next, to determine the time on that day in hours:
Decimal remainder of X * 24hrs/day = Y hours from preceding day, than:
24 – Y hours from preceding day = Y hours on the calendar day
In our example, 5. 95 days was calculated from equation 6b.1. The decimal remainder is .95 from the end of the preceding day so:
.95 days * 24 hrs/day = 22.8 hours from the end of the preceding day, or:
24-22.8 = 1.2 hour on the calendar day, or 1 am (+ some fraction of an hour).
(6b.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 6b.3 and the decimal remainder was 0.2, so:
0.2 * 60 = 12 minutes past the hour or 1:12 am
(6b.5) Combine 6b through 6b.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
(6b.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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