Time period filter has a problem with formatting timestamp to local datetime format.
I've translated default format: 'd. m. Y H:i' http://www.zabbix.org/pootle/sk/Zabbix-trunk/LC_MESSAGES/frontend.po/translate/#search=d.%20m.%20Y%20H%3Ai&sfields=source,target
Another languages are OK. They don't have translated format or they have standard English format 'Y-m-d H:i' - it can be also workaround.
Probably function format of CDate class has some bug. Problem tested on current version of Firefox and Internet Explorer and rev 46285 (svn checkout):
Problem code:
setTabInfo: function() {
var period = timeControl.timeline.period(),
usertime = timeControl.timeline.usertime();
// secure browser from incorrect user actions
if (isNaN(period) || isNaN(usertime))

{ return; }

this.dom.info_period.innerHTML = formatTimestamp(period, false, true);
// info left
var userStartTime = usertime - period;
this.dom.info_left.innerHTML = new CDate(userStartTime * 1000).format(locale['S_DATE_FORMAT']);
this.dom.info_left.setAttribute('data-timestamp', userStartTime);
// info right
var right_info = new CDate(usertime * 1000).format(locale['S_DATE_FORMAT']);
this.dom.info_right.setAttribute('data-timestamp', usertime);
if (timeControl.timeline.now())

{ right_info += ' (' + locale['S_NOW_SMALL'] + '!) '; }

this.dom.info_right.innerHTML = right_info;
// seting zoom link styles
this.setZoomLinksStyle();
},