var s = "2000.12.31 - 00:00:00".match(/^(\d{4})\.(\d{2})\.(\d{2}) - (\d{2}):(\d{2}):(\d{2})$/);
var d = new Date(+s[1],+s[2]-1,+s[3],+s[4],+s[5],+s[6]);
d.setDate( d.getDate()+1 );
d.toISOString().replace(/^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2}).*$/,'$1.$2.$3 - $4:$5:$6');

function a(str){
    var code = str.charCodeAt(0);
    for(var i=1;i<str.length;i++){
        if(str.charCodeAt(i)-code != i){
            return String.fromCharCode(code+i);
        }
    }
}
['abcde','abce','абвгд','авгд'].forEach(s=>{
    console.log(`a(${s}) = `+a(s));
});

a(abcde) = undefined
a(abce) = d
a(абвгд) = undefined
a(авгд) = б