PHYS 3154 Observational Astrophysics J. H. Simonetti jsimonetti@vt.edu

How to Convert UT Dates and Times into Julian Dates in Excel

All dates/times of observations in this course should be in UT, or Julian Date. The Julian Date is best for data on variable stars, which may be compared with data on the same stars from different dates. This page describes how to convert a UT Date and UT Time into a Julian Date in Excel. The conversion can be used on large columns of such dates and times.

I know this conversion works in my Excel program (Microsoft Excel 2002 SP3). It may also work in other versions of Excel. You can test its results in your version of Excel using the Julian Date Converter at the US Naval Observatory website.

The Conversion

For a UT date and UT time given in cells A1 and B1, the Julian Date can be put into another cell by typing the following formula into that cell:

= A1 + 2415018.5 + B1 - INT(B1)

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References:
Microsoft Office Assistance: Insert Julian dates
Microsoft Office Assistance: Convert times

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