Use Flask-Babel to localize datetime in the History Page

By formatting datetime on the server, we get nice localized datetime
strings that are adapted to the currently-selected language.  Example:

- English: "Apr 26, 2020, 3:58:54 PM"
- French: "26 avr. 2020 à 15:58:54"
- German: "26.04.2020, 15:58:54"
- Spanish: "26 abr. 2020 15:58:54"
- Indonesian: "26 Apr 2020 15.58.54"
- Chinese: "2020年4月26日 下午3:58:54"

However, there is a downside: time is not adapted to the user timezone.

The solution is to define a timezone on the server: we use the server OS
timezone by default, and it can be customized through the
BABEL_DEFAULT_TIMEZONE setting.  It's still not ideal, because it assumes
that all users are in the same timezone (the one configured on the server).

3 files changed, 7 insertions, 4 deletions

diff --git a/ihatemoney/history.py b/ihatemoney/history.py
index 801e39e..3f99420 100644
--- a/ihatemoney/history.py
+++ b/ihatemoney/history.py
@@ -82,7 +82,7 @@ def get_history(project, human_readable_names=True):
                 object_str = describe_version(version)
 
             common_properties = {
-                "time": version.transaction.issued_at.strftime("%Y-%m-%dT%H:%M:%SZ"),
+                "time": version.transaction.issued_at,
                 "operation_type": version.operation_type,
                 "object_type": object_type,
                 "object_desc": object_str,
diff --git a/ihatemoney/run.py b/ihatemoney/run.py
index 0f32dfb..1d2a575 100644
--- a/ihatemoney/run.py
+++ b/ihatemoney/run.py
@@ -2,6 +2,7 @@ import os
 import os.path
 import warnings
 
+from babel.dates import LOCALTZ
 from flask import Flask, g, render_template, request, session
 from flask_babel import Babel, format_currency
 from flask_mail import Mail
@@ -150,8 +151,10 @@ def create_app(
     app.jinja_env.globals["locale_from_iso"] = locale_from_iso
     app.jinja_env.filters["minimal_round"] = minimal_round
 
-    # Translations
-    babel = Babel(app)
+    # Translations and time zone (used to display dates).  The timezone is
+    # taken from the BABEL_DEFAULT_TIMEZONE settings, and falls back to
+    # the local timezone of the server OS by using LOCALTZ.
+    babel = Babel(app, default_timezone=str(LOCALTZ))
 
     # Undocumented currencyformat filter from flask_babel is forwarding to Babel format_currency
     # We overwrite it to remove the currency sign ¤ when there is no currency
diff --git a/ihatemoney/templates/history.html b/ihatemoney/templates/history.html
index d9f2b4d..d4965d8 100644
--- a/ihatemoney/templates/history.html
+++ b/ihatemoney/templates/history.html
@@ -161,7 +161,7 @@
     <tbody>
     {% for event in history %}
     <tr>
-            <td><script>document.write(localizeTime("{{ event.time }}"));</script></td>
+            <td>{{ event.time|datetimeformat("medium") }}</td>
             <td >
                 <div class="history_icon">
                     <i {% if event.operation_type == OperationType.INSERT  %}