diff --git a/web_app/js/external/sprintf.js b/web_app/js/external/sprintf.js
new file mode 100644
index 0000000..37f2d62
--- /dev/null
+++ b/web_app/js/external/sprintf.js
@@ -0,0 +1,208 @@
+(function(window) {
+    var re = {
+        not_string: /[^s]/,
+        number: /[diefg]/,
+        json: /[j]/,
+        not_json: /[^j]/,
+        text: /^[^\x25]+/,
+        modulo: /^\x25{2}/,
+        placeholder: /^\x25(?:([1-9]\d*)\$|\(([^\)]+)\))?(\+)?(0|'[^$])?(-)?(\d+)?(?:\.(\d+))?([b-gijosuxX])/,
+        key: /^([a-z_][a-z_\d]*)/i,
+        key_access: /^\.([a-z_][a-z_\d]*)/i,
+        index_access: /^\[(\d+)\]/,
+        sign: /^[\+\-]/
+    }
+
+    function sprintf() {
+        var key = arguments[0], cache = sprintf.cache
+        if (!(cache[key] && cache.hasOwnProperty(key))) {
+            cache[key] = sprintf.parse(key)
+        }
+        return sprintf.format.call(null, cache[key], arguments)
+    }
+
+    sprintf.format = function(parse_tree, argv) {
+        var cursor = 1, tree_length = parse_tree.length, node_type = "", arg, output = [], i, k, match, pad, pad_character, pad_length, is_positive = true, sign = ""
+        for (i = 0; i < tree_length; i++) {
+            node_type = get_type(parse_tree[i])
+            if (node_type === "string") {
+                output[output.length] = parse_tree[i]
+            }
+            else if (node_type === "array") {
+                match = parse_tree[i] // convenience purposes only
+                if (match[2]) { // keyword argument
+                    arg = argv[cursor]
+                    for (k = 0; k < match[2].length; k++) {
+                        if (!arg.hasOwnProperty(match[2][k])) {
+                            throw new Error(sprintf("[sprintf] property '%s' does not exist", match[2][k]))
+                        }
+                        arg = arg[match[2][k]]
+                    }
+                }
+                else if (match[1]) { // positional argument (explicit)
+                    arg = argv[match[1]]
+                }
+                else { // positional argument (implicit)
+                    arg = argv[cursor++]
+                }
+
+                if (get_type(arg) == "function") {
+                    arg = arg()
+                }
+
+                if (re.not_string.test(match[8]) && re.not_json.test(match[8]) && (get_type(arg) != "number" && isNaN(arg))) {
+                    throw new TypeError(sprintf("[sprintf] expecting number but found %s", get_type(arg)))
+                }
+
+                if (re.number.test(match[8])) {
+                    is_positive = arg >= 0
+                }
+
+                switch (match[8]) {
+                    case "b":
+                        arg = arg.toString(2)
+                    break
+                    case "c":
+                        arg = String.fromCharCode(arg)
+                    break
+                    case "d":
+                    case "i":
+                        arg = parseInt(arg, 10)
+                    break
+                    case "j":
+                        arg = JSON.stringify(arg, null, match[6] ? parseInt(match[6]) : 0)
+                    break
+                    case "e":
+                        arg = match[7] ? arg.toExponential(match[7]) : arg.toExponential()
+                    break
+                    case "f":
+                        arg = match[7] ? parseFloat(arg).toFixed(match[7]) : parseFloat(arg)
+                    break
+                    case "g":
+                        arg = match[7] ? parseFloat(arg).toPrecision(match[7]) : parseFloat(arg)
+                    break
+                    case "o":
+                        arg = arg.toString(8)
+                    break
+                    case "s":
+                        arg = ((arg = String(arg)) && match[7] ? arg.substring(0, match[7]) : arg)
+                    break
+                    case "u":
+                        arg = arg >>> 0
+                    break
+                    case "x":
+                        arg = arg.toString(16)
+                    break
+                    case "X":
+                        arg = arg.toString(16).toUpperCase()
+                    break
+                }
+                if (re.json.test(match[8])) {
+                    output[output.length] = arg
+                }
+                else {
+                    if (re.number.test(match[8]) && (!is_positive || match[3])) {
+                        sign = is_positive ? "+" : "-"
+                        arg = arg.toString().replace(re.sign, "")
+                    }
+                    else {
+                        sign = ""
+                    }
+                    pad_character = match[4] ? match[4] === "0" ? "0" : match[4].charAt(1) : " "
+                    pad_length = match[6] - (sign + arg).length
+                    pad = match[6] ? (pad_length > 0 ? str_repeat(pad_character, pad_length) : "") : ""
+                    output[output.length] = match[5] ? sign + arg + pad : (pad_character === "0" ? sign + pad + arg : pad + sign + arg)
+                }
+            }
+        }
+        return output.join("")
+    }
+
+    sprintf.cache = {}
+
+    sprintf.parse = function(fmt) {
+        var _fmt = fmt, match = [], parse_tree = [], arg_names = 0
+        while (_fmt) {
+            if ((match = re.text.exec(_fmt)) !== null) {
+                parse_tree[parse_tree.length] = match[0]
+            }
+            else if ((match = re.modulo.exec(_fmt)) !== null) {
+                parse_tree[parse_tree.length] = "%"
+            }
+            else if ((match = re.placeholder.exec(_fmt)) !== null) {
+                if (match[2]) {
+                    arg_names |= 1
+                    var field_list = [], replacement_field = match[2], field_match = []
+                    if ((field_match = re.key.exec(replacement_field)) !== null) {
+                        field_list[field_list.length] = field_match[1]
+                        while ((replacement_field = replacement_field.substring(field_match[0].length)) !== "") {
+                            if ((field_match = re.key_access.exec(replacement_field)) !== null) {
+                                field_list[field_list.length] = field_match[1]
+                            }
+                            else if ((field_match = re.index_access.exec(replacement_field)) !== null) {
+                                field_list[field_list.length] = field_match[1]
+                            }
+                            else {
+                                throw new SyntaxError("[sprintf] failed to parse named argument key")
+                            }
+                        }
+                    }
+                    else {
+                        throw new SyntaxError("[sprintf] failed to parse named argument key")
+                    }
+                    match[2] = field_list
+                }
+                else {
+                    arg_names |= 2
+                }
+                if (arg_names === 3) {
+                    throw new Error("[sprintf] mixing positional and named placeholders is not (yet) supported")
+                }
+                parse_tree[parse_tree.length] = match
+            }
+            else {
+                throw new SyntaxError("[sprintf] unexpected placeholder")
+            }
+            _fmt = _fmt.substring(match[0].length)
+        }
+        return parse_tree
+    }
+
+    var vsprintf = function(fmt, argv, _argv) {
+        _argv = (argv || []).slice(0)
+        _argv.splice(0, 0, fmt)
+        return sprintf.apply(null, _argv)
+    }
+
+    /**
+     * helpers
+     */
+    function get_type(variable) {
+        return Object.prototype.toString.call(variable).slice(8, -1).toLowerCase()
+    }
+
+    function str_repeat(input, multiplier) {
+        return Array(multiplier + 1).join(input)
+    }
+
+    /**
+     * export to either browser or node.js
+     */
+    if (typeof exports !== "undefined") {
+        exports.sprintf = sprintf
+        exports.vsprintf = vsprintf
+    }
+    else {
+        window.sprintf = sprintf
+        window.vsprintf = vsprintf
+
+        if (typeof define === "function" && define.amd) {
+            define(function() {
+                return {
+                    sprintf: sprintf,
+                    vsprintf: vsprintf
+                }
+            })
+        }
+    }
+})(typeof window === "undefined" ? this : window);
\ No newline at end of file
diff --git a/web_app/js/sif-tools.js b/web_app/js/sif-tools.js
index 39278a0..15d64b4 100644
--- a/web_app/js/sif-tools.js
+++ b/web_app/js/sif-tools.js
@@ -50,6 +50,9 @@ function setup_ui_elements()
 		}
 	});
 
+    // set up datepicker
+    $( "#gem_desired_date" ).datepicker();
+    
 	// set up buttons
 	["calculate-rank", "reset-rank", "calculate-gems", "reset-gems"].forEach(function(entry) {
 		var selector = "#button-" + entry;
@@ -206,10 +209,83 @@ function handle_gem_mode_select()
     }
 }
 
+function is_muse_members_birthday(moment)
+{
+    var the_month = month(moment);
+    var the_day = day(moment);
+
+    var is_bday = false;
+    var bday_name = "";
+    if (the_month == 1 && the_day == 17) {
+        is_bday = true;
+        bday_name = "Hanayo";
+    } else if (the_month == 3 && the_day == 15) {
+        is_bday = true;
+        bday_name = "Umi";
+    } else if (the_month == 4 && the_day == 19) {
+        is_bday = true;
+        bday_name = "Maki";
+    } else if (the_month == 6 && the_day == 9) {
+        is_bday = true;
+        bday_name = "Nozomi";
+    } else if (the_month == 7 && the_day == 22) {
+        is_bday = true;
+        bday_name = "Nico";
+    } else if (the_month == 8 && the_day == 3) {
+        is_bday = true;
+        bday_name = "Honoka";
+    } else if (the_month == 9 && the_day == 12) {
+        is_bday = true;
+        bday_name = "Kotori";
+    } else if (the_month == 10 && the_day == 21) {
+        is_bday = true;
+        bday_name = "Eli";
+    } else if (the_month == 11 && the_day == 1) {
+        is_bday = true;
+        bday_name = "Rin";
+    }
+    return [is_bday, bday_name];
+}
+
+function is_gem_day(moment)
+{
+    var the_day = day(moment);
+    // according the login bonus chart, gems are given out on days numbered 1,6,11,16,21,26,30
+    if (the_day == 1 || the_day == 6 || the_day == 11 || the_day == 16 || the_day == 21 || the_day == 26 || the_day == 30) {
+        return true;
+    }
+    return false;
+}
+
+// moment.js starts months/days/years at 0... weird...
+function month(moment)
+{
+    return moment.month() + 1;
+}
+
+// moment.js uses date() not day()
+function day(moment)
+{
+    return moment.date();
+}
+
+function year(moment)
+{
+    return moment.year();
+}
+
+function is_same_day(m1, m2)
+{
+    if (month(m1) == month(m2) && day(m1) == day(m2) && year(m1) == year(m2)) {
+        return true;
+    }
+    return false;
+}
+
 function calculate_gems()
 {
     var verbose = $("#gems_verbose").is(':checked');
-    var current_gems = $("#current_gems").val();
+    var current_gems = parseInt($("#current_gems").val());
     if (isNaN(current_gems)) {
         alert("Error: invalid number of current gems. Please check your input and try again.");
         return;
@@ -217,13 +293,59 @@ function calculate_gems()
     var mode = $("input[name=gem-mode]:checked").val();
     if (mode === "DATE")  {
         var target_date = $("#gem_desired_date").val();
+        var target_date_object = moment(new Date(target_date));
+
+        var now = moment(new Date());
+        if (target_date_object.isBefore(now) || is_same_day(now, target_date_object)) {
+            window.alert("Error: the date must be in the future.");
+            return;
+        }
+        
+        // ready to rock
+        var resultsString = sprintf("Today is %02d/%02d/%04d and you currently have %d love gems.<br />(Assuming you collected any gems you got today and already counted those.)", month(now), day(now), year(now), current_gems);
+        var verboseText = "";
+        var gems = current_gems
+        now = now.add(1, 'days')
+        while (now.isBefore(target_date_object) || is_same_day(now, target_date_object)) {
+            // is it a login bonus?
+            if (is_gem_day(now)) {
+                gems += 1;
+            }
+            
+            // is it a birthday?
+            var birthday_tuple = is_muse_members_birthday(now);
+            var is_bday = birthday_tuple[0];
+            var name = birthday_tuple[1];
+            if (is_bday) {
+                gems += 5;
+            }
+            
+            // record verbose output if desired
+            if (verbose) {
+                if (is_gem_day(now) && is_bday) {
+                    verboseText += sprintf("%02d/%02d/%04d: free gem as login bonus AND it's %s's birthday! You get 6 gems, which brings you to %d gems.\n", month(now), day(now), year(now), name, gems);
+                }
+                
+                if (is_bday && !is_gem_day(now)) {
+                    verboseText += sprintf("%02d/%02d/%04d: it's %s's birthday! You get 5 gems, which brings you to %d gems.\n", month(now), day(now), year(now), name, gems);
+                }
+                
+                if (is_gem_day(now) && !is_bday) {
+                    verboseText = verboseText + sprintf("%02d/%02d/%04d: free gem as login bonus, which brings you to %d gems.\n", month(now), day(now), year(now), gems);
+                }
+            }
+
+            now = now.add(1, 'days')
+        }
+
+        resultsString = resultsString + sprintf("<br />You will have %d love gems on %02d/%02d/%04d. Good things come to those who wait!", gems, month(target_date_object), day(target_date_object), year(target_date_object));
+        $("#gem-result-summary").html(resultsString);
         if (verbose) {
-            $("#gem-result-summary").text("Insert verbose results here");
-            $("#gem-result-verbose-area").val("VERBOSITY IS GOOD!");
+            $("#gem-result-verbose-area").val(verboseText);
             $("#gem-result-textarea").show();
         } else {
             $("#gem-result-summary").text("Insert non-verbose results here");
-            $("#gem-result-verbose-area").val("VERBOSITY IS BAD!");
+            $("#gem-result-verbose-area").val(verboseText);
             $("#gem-result-textarea").hide();
         }
     } else if (mode === "GEMS")  {
diff --git a/web_app/sif_tools.html b/web_app/sif_tools.html
index 4be843c..f49ac41 100644
--- a/web_app/sif_tools.html
+++ b/web_app/sif_tools.html
@@ -8,6 +8,7 @@
   <script src="js/external/jquery.js"></script>
   <script src="js/external/jquery-ui.js"></script>
   <script src="js/external/moment.js"></script>
+  <script src="js/external/sprintf.js"></script>
   <script src="js/sif-tools.js"></script>
 </head>
 <body>