how to use a variable's value as attribute's value?

[creativegamer03]

tried to add a date and time value formatted as "YYYY-MM-DD HH:MM:SS" to an attribute.
here's two snippets from my code (its a simple sample program):

...
time_t curr_time;
    tm * curr_tm;
    char date_string[100];
    char time_string[100];

    std::time(&curr_time);
    curr_tm = std::localtime(&curr_time);

    std::strftime(date_string, 50, "%B %d, %Y", curr_tm);
    std::strftime(time_string, 50, "%T", curr_tm);
    
    std::string now = std::string() + date_string + time_string + "+08:00";
...
main_root.append_attribute("height") = 1080;
    main_root.append_attribute("width") = 1920;
    main_root.append_attribute("docType") = 0;
    main_root.append_attribute("versionNumber") = 600;
    main_root.append_attribute("usedCount") = 0;
    main_root.append_attribute("backgroundColor") = "0 0 0 1";
    main_root.append_attribute("drawingBackgroundColor") = false;
    main_root.append_attribute("CCLIDisplay") = false;
    main_root.append_attribute("lastDateUsed") = now;
    main_root.append_attribute("slectedArrangementID") = "";
...

error I get:

D:\_a\xml_test\xml_test.cpp:49:50: error: no match for 'operator=' (operand types are 'pugi::xml_attribute' and 'std::__cxx11::string {aka std::__cxx11::basic_string<char>}')
     main_root.append_attribute("lastDateUsed") = a;
                                                  ^

how?

[zeux]

    main_root.append_attribute("lastDateUsed") = now.c_str();