Calculating time spans with datetime2

Question

With datetime2, is there a way to calculate an end time from a start time if I have the duration in minutes? Something like

\newcommand*\timespan{90}
\newcommand*\starttime{13:45:00}
\DTMtime{\starttime+\timespan}

(analoguous to \DTMdate where you can add days)

I don't see anything in the docs, but I have the impression it might be possible via \DTMfetchhour and \DTMfetchminute.

(The background here is changing start time with a fixed duration, and I want to automatically output the whole slot including the end time)

Answer 1

This is a preliminary version, using expl3 features and calculating the total number of seconds, then splitting and calculating the new time.

It does not indicate that the time span is more than 24 hours, so using a timespan of multiples of 1440 (minutes) will result in the same time as the start time.

Wrapping hour values after midnight is done.

datetime2 does not provide macros in order to calculate time spans or end times in sense of hour:minute:seconds -- there are some computation macros concerning dates.

\documentclass{article}

\usepackage{xparse}
\usepackage{datetime2}

\newcommand*\timespan{90}
\newcommand*\starttime{13:45:00}

\ExplSyntaxOn
\cs_generate_variant:Nn \int_set:Nn {NV}
\cs_generate_variant:Nn \seq_set_split:Nnn {NVn,NVV}

\NewDocumentCommand{\endtime}{+m+m}{%
  \group_begin:
  \tl_set:Nx \l_tmpa_tl {#1}
  \seq_set_split:NVV \l_tmpa_seq {\c_colon_str} \l_tmpa_tl
  \exp_args:NNx \fp_set:Nn \l_tmpa_fp {\seq_item:Nn \l_tmpa_seq {1} * 3600 + \seq_item:Nn \l_tmpa_seq {2} * 60 + \seq_item:Nn \l_tmpa_seq {3}  }
  \exp_args:NNx \fp_set:Nn \l_tmpb_fp {#2 * 60}
  \fp_add:Nn \l_tmpa_fp {#2 * 60} 
  \tl_clear:N \l_tmpa_tl 
  % Compute hours
  \exp_args:NNx \int_set:Nn \l_tmpa_int {\int_div_truncate:nn {\fp_use:N \l_tmpa_fp}{3600}}
  \int_compare:nNnTF {\l_tmpa_int } > {23} {% After midnight?
    \exp_args:NNx \int_set:Nn \l_tmpb_int {\int_mod:nn {\l_tmpa_int } {24}}
  }{%
    \int_set_eq:NN \l_tmpb_int \l_tmpa_int
  }
  \tl_put_right:NV \l_tmpa_tl {\l_tmpb_int\c_colon_str}

  % Remaining seconds, split them into minutes and seconds 
  \int_set:Nn \l_tmpa_int {\fp_use:N \l_tmpa_fp}

  \exp_args:NNx \int_set:Nn \l_tmpb_int {\int_mod:nn {\l_tmpa_int}{3600}}
  \exp_args:NNx \int_set:Nn \l_tmpb_int {\int_div_truncate:nn {\l_tmpb_int}{60}}
  \tl_put_right:NV \l_tmpa_tl {\l_tmpb_int\c_colon_str}
  % Finally splitting: Seconds
  \exp_args:NNx \int_set:Nn \l_tmpa_int {\int_mod:nn {\l_tmpa_int}{60}}
  \tl_put_right:NV \l_tmpa_tl {\l_tmpa_int}
  % Display output:
  \exp_args:NV \DTMtime{\l_tmpa_tl}
  \group_end:
}



\ExplSyntaxOff

\begin{document}
We started at \expandafter\DTMtime\expandafter{\starttime} 

The event lasts \timespan\ minutes

It ends at \endtime{\starttime}{\timespan}
\end{document}