Before the drag and drop,
LAST-MODIFIED:20120427T115654Z
DTSTART;TZID=Europe/Paris:20120501T110000

---------------------------------------------------------------------------
BEGIN:VCALENDAR
PRODID:-//EDeal SARL./SOGo 1.3.11//EN
VERSION:2.0
CALSCALE:GREGORIAN
BEGIN:VEVENT
DTSTAMP:20120427T095513Z
UID:f2fcd462-75e5-4d8e-979b-6e25ec53cb21
CLASS:PUBLIC
DTSTART;TZID=Europe/Paris:20120501T110000
DTEND;TZID=Europe/Paris:20120501T120000
SUMMARY:int&
SEQUENCE:2
ATTENDEE;PARTSTAT=NEEDS-ACTION;ROLE=REQ-PARTICIPANT;RSVP=TRUE;CN=Adrien POU
 PARD:mailto:nomail0000000000008aaf
ATTENDEE;PARTSTAT=NEEDS-ACTION;ROLE=REQ-PARTICIPANT;RSVP=TRUE:mailto:edeal@
 acacs.org
ORGANIZER;CN=Vincent Test:mailto:vincent@jolo.fr
LAST-MODIFIED:20120427T115654Z
TRANSP:OPAQUE
END:VEVENT
BEGIN:VTIMEZONE
TZID:Europe/Paris
X-LIC-LOCATION:Europe/Paris
BEGIN:DAYLIGHT
TZOFFSETFROM:+0100
TZOFFSETTO:+0200
TZNAME:CEST
DTSTART:19700329T020000
RRULE:BYDAY=-1SU;FREQ=YEARLY;BYMONTH=3
END:DAYLIGHT
BEGIN:STANDARD
TZOFFSETFROM:+0200
TZOFFSETTO:+0100
TZNAME:CET
DTSTART:19701025T030000
RRULE:BYDAY=-1SU;FREQ=YEARLY;BYMONTH=10
END:STANDARD
END:VTIMEZONE
END:VCALENDAR
===========================================================================
===========================================================================
===========================================================================

After Drag and drop, 
DTSTART;TZID=Europe/Paris:20120503T110000 => updated
LAST-MODIFIED:20120427T115654Z => not updated
---------------------------------------------------------------------------
BEGIN:VCALENDAR
PRODID:-//EDeal SARL./SOGo 1.3.11//EN
VERSION:2.0
CALSCALE:GREGORIAN
BEGIN:VEVENT
DTSTAMP:20120427T095513Z
UID:f2fcd462-75e5-4d8e-979b-6e25ec53cb21
CLASS:PUBLIC
DTSTART;TZID=Europe/Paris:20120503T110000
DTEND;TZID=Europe/Paris:20120503T120000
SUMMARY:int&
SEQUENCE:4
ATTENDEE;PARTSTAT=NEEDS-ACTION;ROLE=REQ-PARTICIPANT;RSVP=TRUE;CN=Adrien POU
 PARD:mailto:nomail0000000000008aaf
ATTENDEE;PARTSTAT=NEEDS-ACTION;ROLE=REQ-PARTICIPANT;RSVP=TRUE:mailto:edeal@
 acacs.org
ORGANIZER;CN=Vincent Test:mailto:vincent@jolo.fr
LAST-MODIFIED:20120427T115654Z
TRANSP:OPAQUE
END:VEVENT
BEGIN:VTIMEZONE
TZID:Europe/Paris
X-LIC-LOCATION:Europe/Paris
BEGIN:DAYLIGHT
TZOFFSETFROM:+0100
TZOFFSETTO:+0200
TZNAME:CEST
DTSTART:19700329T020000
RRULE:BYDAY=-1SU;FREQ=YEARLY;BYMONTH=3
END:DAYLIGHT
BEGIN:STANDARD
TZOFFSETFROM:+0200
TZOFFSETTO:+0100
TZNAME:CET
DTSTART:19701025T030000
RRULE:BYDAY=-1SU;FREQ=YEARLY;BYMONTH=10
END:STANDARD
END:VTIMEZONE
END:VCALENDAR