Utility to convert dates and/or times to a continuous time unit for analysis. The program expects one or two columns, each containing dates or times. If both are given, then time is added to date to give the final time value.
Dates can be given in the formats Year/Month/Day or Day/Month/Year. Years need all digits (a year given as 11 will mean 11 AD, not 2011). Only Gregorian calendar dates are supported. Leap years are taken into account.
Time can be given as Hours:Minutes or Hours:Minutes:Seconds (seconds can include decimals).
The output units can be years (using the Gregorian mean year of 365.2425 days), days (of 86400 seconds), hours, minutes or seconds.
The starting time (time zero) can be the smallest given time, the beginning of the first day, the beginning of the first year, year 0 (note the “astronomical” convention where the year before year 1 is year 0), or the beginning of the first Julian day (noon, year -4712).
The program operates with simple (UT) time, defined with respect to the Earth’s rotation and with a fixed number of seconds (86400) per day.
If your input data consist of space-separated date-time values, such as “2011/12/24 18:00:00.00”, then you may have to use the “Import text file” function to read the data such that dates and times are split into separate columns.
The calculation of Julian day (which is used to find number of days between two dates) follows Meeus (1991):
if month <= 2 begin year := year - 1; month := month + 12; end;
A = floor(year/100);
B = 2 – A + floor(A/4);
JD = floor(365.25(year + 4716)) + floor(30.6001(month+1)) + day + B – 1524.5;
Reference
Meeus, J. 1991. Astronomical algorithms. Willmann-Bell, Richmond.