Render exercise deadlines in the configured system timezone

The edit-configuration form previously dumped the naive-UTC deadline
straight into a datetime-local input, so admins saw UTC wall-clock while
the SSH banner and scoreboard (which route through utc_datetime_to_local_tz)
displayed the system-timezone value.

Convert naive-UTC to the system timezone on the GET path, render the
admin "View Exercise" deadline fields through a new localdt Jinja filter,
and replace the native datetime-local picker with a flatpickr instance
(24h, Monday week start, altInput for a friendly display, defaultDate/
defaultHour/defaultMinute pre-populated from the current value). Init
runs on window.load so bootstrap-material-design's form-group wrapping
does not clobber flatpickr.

Author: Nils Bars

tests/unit/test_util.py

@@ -4,15 +4,19 @@
 Tests for utility functions that don't require Flask/DB context.
 """
 
+import datetime
+
 import pytest
 from unittest.mock import MagicMock, patch
 from colorama import Fore, Style
 
 from ref.core.util import (
     AnsiColorUtil,
+    datetime_to_string,
     is_db_serialization_error,
     is_deadlock_error,
     ssh_key_basename,
+    utc_datetime_to_local_tz,
 )
 
 
@@ -268,3 +272,57 @@ def test_leading_whitespace_is_stripped(self):
 
     def test_unknown_algo_falls_back_to_rsa(self):
         assert ssh_key_basename("bogus-algo AAAA") == "id_rsa"
+
+
+@pytest.mark.offline
+class TestUtcDatetimeToLocalTz:
+    """Conversion of naive-UTC datetimes (as stored in the DB) to the
+    configured system timezone, for display purposes."""
+
+    def test_converts_naive_utc_to_berlin_cest(self):
+        naive_utc = datetime.datetime(2026, 4, 26, 15, 59)
+        with patch("ref.core.util.SystemSettingsManager") as ssm:
+            ssm.TIMEZONE.value = "Europe/Berlin"
+            local = utc_datetime_to_local_tz(naive_utc)
+        assert local.strftime("%Y-%m-%dT%H:%M") == "2026-04-26T17:59"
+
+    def test_converts_naive_utc_to_berlin_cet_in_winter(self):
+        naive_utc = datetime.datetime(2026, 1, 15, 12, 0)
+        with patch("ref.core.util.SystemSettingsManager") as ssm:
+            ssm.TIMEZONE.value = "Europe/Berlin"
+            local = utc_datetime_to_local_tz(naive_utc)
+        assert local.strftime("%Y-%m-%dT%H:%M") == "2026-01-15T13:00"
+
+    def test_utc_timezone_is_identity(self):
+        naive_utc = datetime.datetime(2026, 4, 26, 15, 59)
+        with patch("ref.core.util.SystemSettingsManager") as ssm:
+            ssm.TIMEZONE.value = "UTC"
+            local = utc_datetime_to_local_tz(naive_utc)
+        assert local.strftime("%Y-%m-%dT%H:%M") == "2026-04-26T15:59"
+
+    def test_crosses_day_boundary(self):
+        naive_utc = datetime.datetime(2026, 4, 26, 23, 30)
+        with patch("ref.core.util.SystemSettingsManager") as ssm:
+            ssm.TIMEZONE.value = "Europe/Berlin"
+            local = utc_datetime_to_local_tz(naive_utc)
+        assert local.strftime("%Y-%m-%dT%H:%M") == "2026-04-27T01:30"
+
+
+@pytest.mark.offline
+class TestDatetimeToString:
+    """Human-readable formatting of DB-stored (naive UTC) datetimes."""
+
+    def test_naive_utc_formatted_in_system_tz(self):
+        naive_utc = datetime.datetime(2026, 4, 26, 15, 59, 0)
+        with patch("ref.core.util.SystemSettingsManager") as ssm:
+            ssm.TIMEZONE.value = "Europe/Berlin"
+            s = datetime_to_string(naive_utc)
+        assert s == "26/04/2026 17:59:00"
+
+    def test_aware_datetime_preserves_its_tz(self):
+        from dateutil import tz as _tz
+
+        aware_utc = datetime.datetime(2026, 4, 26, 15, 59, 0, tzinfo=_tz.gettz("UTC"))
+        # Aware datetimes pass through without re-conversion.
+        s = datetime_to_string(aware_utc)
+        assert s == "26/04/2026 15:59:00"

webapp/ref/__init__.py

@@ -389,11 +389,19 @@ def ansi2html_filter(s):
         ret = ansi2html.Ansi2HTMLConverter().convert(s, full=False)
         return ret
 
+    from ref.core.util import datetime_to_string
+
+    def localdt(ts):
+        if ts is None:
+            return "—"
+        return datetime_to_string(ts)
+
     app.jinja_env.filters["quote_plus"] = lambda u: urllib.parse.quote_plus(u)
     app.jinja_env.filters["any"] = any
     app.jinja_env.filters["all"] = all
     app.jinja_env.filters["not"] = lambda e: [not x for x in e]
     app.jinja_env.filters["ansi2html"] = ansi2html_filter
+    app.jinja_env.filters["localdt"] = localdt
 
     def syntax_highlight(val):
         try:

webapp/ref/templates/exercise_config_edit.html

@@ -1,6 +1,11 @@
 {% extends "admin_base.html" %}
 {% import "wtf_bootstrap_utils.html" as wtf_utils %}
 
+{% block head %}
+<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/flatpickr@4.6.13/dist/flatpickr.min.css">
+<script src="https://cdn.jsdelivr.net/npm/flatpickr@4.6.13/dist/flatpickr.min.js"></script>
+{% endblock %}
+
 {% block title %}
 Edit Exercise: {{ short_name }}
 {% endblock %}
@@ -25,10 +30,16 @@ <h4 class="card-header">
 
             <div class="row mb-3">
                 <div class="col-md-6">
-                    {{ wtf_utils.render_field(form.submission_deadline_start, "datetime-local") }}
+                    {{ wtf_utils.render_field(form.submission_deadline_start, "text") }}
+                    <small class="form-text text-muted">
+                        Interpreted as {{ settings.TIMEZONE.value }} (system timezone).
+                    </small>
                 </div>
                 <div class="col-md-6">
-                    {{ wtf_utils.render_field(form.submission_deadline_end, "datetime-local") }}
+                    {{ wtf_utils.render_field(form.submission_deadline_end, "text") }}
+                    <small class="form-text text-muted">
+                        Interpreted as {{ settings.TIMEZONE.value }} (system timezone).
+                    </small>
                 </div>
             </div>
 
@@ -328,4 +339,39 @@ <h5>Per-task scoring policies</h5>
     serialize();
 })();
 </script>
+
+<script>
+(function () {
+    function parseIsoLocal(s) {
+        var m = s && s.match(/^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2})$/);
+        if (!m) return null;
+        return new Date(+m[1], +m[2] - 1, +m[3], +m[4], +m[5]);
+    }
+    function init(id) {
+        var el = document.getElementById(id);
+        if (!el) return;
+        var initial = parseIsoLocal(el.value);
+        flatpickr(el, {
+            enableTime: true,
+            time_24hr: true,
+            minuteIncrement: 1,
+            allowInput: true,
+            dateFormat: "Y-m-d\\TH:i",
+            altInput: true,
+            altFormat: "Y-m-d H:i",
+            defaultDate: initial,
+            defaultHour: initial ? initial.getHours() : 23,
+            defaultMinute: initial ? initial.getMinutes() : 59,
+            locale: { firstDayOfWeek: 1 },
+        });
+    }
+    // bootstrap-material-design rewrites form-control inputs on
+    // $(document).ready (wrapping them in .bmd-form-group), which clobbers
+    // flatpickr if we init first. Run on `load` so BMD has finished.
+    window.addEventListener("load", function () {
+        init("submission_deadline_start");
+        init("submission_deadline_end");
+    });
+})();
+</script>
 {% endblock %}

webapp/ref/templates/exercise_view_single.html

@@ -20,9 +20,9 @@ <h5>Meta</h5>
     <dt class="col-sm-3">Submission Tests Enabled</dt>
     <dd class="col-sm-9">{{ exercise.submission_test_enabled }}</dd>
     <dt class="col-sm-3">Deadline Start</dt>
-    <dd class="col-sm-9">{{ moment(exercise.submission_deadline_start).format()  }}</dd>
+    <dd class="col-sm-9">{{ exercise.submission_deadline_start | localdt }} <small class="text-muted">({{ settings.TIMEZONE.value }})</small></dd>
     <dt class="col-sm-3">Deadline End</dt>
-    <dd class="col-sm-9">{{ moment(exercise.submission_deadline_end).format()  }}</dd>
+    <dd class="col-sm-9">{{ exercise.submission_deadline_end | localdt }} <small class="text-muted">({{ settings.TIMEZONE.value }})</small></dd>
     <dt class="col-sm-3">Build Status</dt>
     <dd class="col-sm-9">{{ exercise.build_job_status }}</dd>
     <dt class="col-sm-3">Build Result</dt><dd class="col-sm-9"></dd>

webapp/ref/view/exercise.py

@@ -28,7 +28,11 @@
 )
 from ref.core.logging import get_logger
 from ref.core.security import sanitize_path_is_subdir
-from ref.core.util import datetime_transmute_into_local, redirect_to_next
+from ref.core.util import (
+    datetime_transmute_into_local,
+    redirect_to_next,
+    utc_datetime_to_local_tz,
+)
 from ref.model import Exercise, ExerciseConfig
 from ref.model.enums import ExerciseBuildStatus
 
@@ -503,12 +507,16 @@ def render():
         form.short_name.data = config.short_name
         form.category.data = config.category
         form.submission_deadline_start.data = (
-            config.submission_deadline_start.strftime("%Y-%m-%dT%H:%M")
+            utc_datetime_to_local_tz(config.submission_deadline_start).strftime(
+                "%Y-%m-%dT%H:%M"
+            )
             if config.submission_deadline_start
             else ""
         )
         form.submission_deadline_end.data = (
-            config.submission_deadline_end.strftime("%Y-%m-%dT%H:%M")
+            utc_datetime_to_local_tz(config.submission_deadline_end).strftime(
+                "%Y-%m-%dT%H:%M"
+            )
             if config.submission_deadline_end
             else ""
         )