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time_get<> : I/P from cin is OK; I/P from an istringstream gives an error

SSteven (189)
I have used the time_get<> facet's get_date() method to read a date from the following sources:
(i) cin and
(ii) an istringstream based on a string s whose value has been obtained from getline(cin, s).

The I/P from cin works fine; the date is read properly and printed fine.
However, the I/P from the istringstream gives an error; the date isn't read properly; an error occurs.

Following is the code for the istringstream source: http://coliru.stacked-crooked.com/a/31704818a11d5629

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vector<string> locales {"C"};


/// Get I/P from a string.
void IPFromStr()
{
    cout << "\nI/P from a string ... " << endl;
    
    /// For each locale name
    for (const auto& locs : locales)
    {
        cout << "locale name: " << locs << endl;

        try
        {
            /// Create the locale.
            locale loc {locs};

            /// Read date/time parts from a string.
            ReadDtPartFromStr(loc);
        }
        catch (const exception& e)
        {
            cerr << " Exception: " << e.what()
                 << endl << endl;
        }
    }
}


/// Read date/time parts from a string.
void ReadDtPartFromStr(locale& loc)
{
    /// Get the time_get<> facet.
    const time_get<char>& tg =
              use_facet<time_get<char>> (loc);

    /// I/P string variable for the read date part.
    string dtpart {};
    
    /// output arguments for the time_get<> facet
    struct tm d {};                             /// time
    ios_base::iostate err = ios_base::goodbit;  /// good
    
    getline(cin, dtpart);
    
    cout << " dtpart: " << dtpart << endl;
        
    /// Get an istringstream for the read date part
    istringstream isdtpart {dtpart};
        
    isdtpart.imbue(loc);
        
    istreambuf_iterator<char> frm(isdtpart), end;
    
    /// Read the date part.
    tg.get_date(frm, end,
                isdtpart,
                err,
                &d);

    /// Print the date read.            
    Print(err, d);
}



The Print() function is:

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/// Print the date read.
void Print(ios_base::iostate& err, tm& d)
{
    if (err)
       cout << " error while reading input" << endl;
    else
       cout << " yyyy/mm/dd hh:mm:ss  : " 
            << d.tm_year + 1900 << '/'
            << d.tm_mon + 1     << '/'
            << d.tm_mday        << ' '
            << d.tm_hour        << ':'
            << d.tm_min         << ':'
            << d.tm_sec
            << endl;
}



Given the following cin input:

01/26/2018


, I get the following output:

I/P from a string ... 

locale name: C
 dtpart: 01/26/2018
 error while reading input


The same I/P works properly for similar functions that read directly from cin.

Why does this error occur when we read from an istringstream? Thanks.
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