NGS and other GPS/ GNSS computational services utilize expressing the date and time as a decimal year to four places; e.g., 2021.8507, 2021.8511, etc. and typically, I’ve never questioned how these services arrive at this until this morning when I noticed that these two examples (the first from DPOS, and the second from OPUS) are both supposedly for November 7, 2021 1400 UTC.
Is there any way LibreOffice Calc can help arbitrate this situation?
Many thanks for any rely.
Kelly
Dates in spreadsheets are serial day numbers since day zero 1899-12-30.
Times in spreadsheets are decimal fractions of day numbers.
2021-11-07 14:00 is equal to decimal 44507.58333 (44507 days since 1899-12-30 plus 0.58333 of a day).
@sokol92 - Vlad, I’m not sure if you were asking me or Villeroy. But the closest online converter tool that I found was Decimal Time Units Conversion Calculator
It does what I was hoping it could do, but the problem was it introduced a different answer; i.e., 2021.8511 was converted to 2021, day 310 15:38:09
The expected answer is day 311 14:00 UTC
@kbellis - by default, the addressee is the author of the post. 
To get the result you specified, enter the number 2021.8511 in cell A1, set the date-time format in cell B1 and enter the formula:
=DATE(INT(A1);1;1) + MOD(A1; 1) * DAYSINYEAR(INT(A1))
So the decimal fraction is the decimal fraction of the year? I failed to see why I should lookup technical specifications like DPOS or OPUS to understand the question.
Dear @Villeroy, the question was, of course, addressed to the author of the topic. I apologize for any possible misunderstanding.
My formula explains the answer that was received by the author of the topic using the converter.
I also did not find anything sensible about DPOS and OPUS.
Wonderful! Thank you Vladimir!! This is perfect!
The esoteric context; i.e., OPUS and DPOS, was just in case anybody wondered why or where decimal years were being used. Sorry if it ended up being a distraction. If you’re at all nerdishly curious, DPOS is a service (https://app.javad.com/jca/#/home) offered by JAVAD GNSS to its customers. OPUS is a service (OPUS: the Online Positioning User Service, process your GNSS data in the National Spatial Reference System) offered by NGS to anybody submitting GNSS observations in RINEX format. The reason behind the concern for the date stamp is in order to account for horizontal time dependent positioning, or in other words, tectonic drift.
I started to understand the question after reading your answer. Instead of explaining the input value 2021.123456 @kbellis gave us another brain teaser.