The following code example is taken from the book
C++20 - The Complete Guide by Nicolai M. Josuttis, Leanpub, 2021
The code is licensed under a Creative Commons Attribution 4.0 International License. Creative Commons License

// raw code

#include <chrono>
#include <string>
#include <sstream>
#include <cassert>

// parse year-month-day with optional hour:minute and optional :sec
// - returns a time_point<> of the passed clock (default: system_clock)
// in seconds
template<typename Clock = std::chrono::system_clock>
auto parseDateTime(const std::string& s)
  // return value:
  std::chrono::time_point<Clock, std::chrono::seconds> tp;

  // string stream to read from:
  std::istringstream sstrm{s};  // no string_view support

  auto posColon = s.find(":");
  if (posColon != std::string::npos) {
    if (posColon != s.rfind(":")) {
      // multiple colons:
      std::chrono::from_stream(sstrm, "%F %T", tp);
    else {
      // one colon:
      std::chrono::from_stream(sstrm, "%F %R", tp);
  else {
    // no colon:
    std::chrono::from_stream(sstrm, "%F", tp);

  // handle invalid formats:

  return tp;