% This file is public domain.

% arara: pdflatex
\documentclass{article}

\usepackage[calc]{datetime2}

\begin{document}

\DTMsavejulianday{mydate}{2457023}
\DTMsavedate{mydate2}{2014-06-20}

Date: \DTMusedate{mydate}.

\newcount\myct
\DTMsaveddatetojulianday{mydate}{\myct}

Julian day number: \number\myct.

\DTMsaveddateoffsettojulianday{mydate}{2}{\myct}

Julian day number (offset+2): \number\myct.

Is \texttt{mydate2} (\DTMusedate{mydate2}) before 
\texttt{mydate} (\DTMusedate{mydate})?
\DTMifdate
 {mydate2}
 {at most=
   \DTMfetchyear{mydate}-\DTMfetchmonth{mydate}-\DTMfetchday{mydate}}
 {yes}{no}.

Is \texttt{mydate2} (\DTMusedate{mydate2}) after 
\texttt{mydate} (\DTMusedate{mydate})?
\DTMifdate
 {mydate2}
 {at least=
   \DTMfetchyear{mydate}-\DTMfetchmonth{mydate}-\DTMfetchday{mydate}}
 {yes}{no}.

\DTMsaveddatediff{mydate}{mydate2}{\myct}

\DTMusedate{mydate} is 
\ifnum\myct=0
 the same day as
\else
  \ifnum\myct<0
   \number-\myct\space day\ifnum\myct<-1s\fi\space before
  \else
   \number\myct\space day\ifnum\myct>1s\fi\space after
  \fi
\fi
\DTMusedate{mydate2}.

Original date: 2014-06-3 20:45:00 +06:00

\DTMsaveaszulutime{mydate}{2014}{6}{3}{20}{45}{0}{6}{0}
UTC+00:00 date: \DTMuse{mydate}.

\DTMsavetimestamp{mydate}{2014-05-01T03:55:00 -06:00}
Original date: \DTMuse{mydate}.

\DTMtozulu{mydate}{mydate2}
UTC+00:00: \DTMuse{mydate2}.

Offset: \DTMdate{2015-03-30+4}.

Last: \DTMdate{2015-02-last}.

Last with offset: \DTMdate{2015-02-last+-2}.

Saved date with offset
\DTMsavedate{offset}{2015-02-last+-2}

Use date: \DTMusedate{offset}.

\end{document}