Hi,
This problem didn't happen before, it seems Joomla tries to convert the dates to the user's timezones before saving them.
To fix this you can modify components/com_cmgroupbuying/models/forms/deal.xml, replace
<field
name="start_date"
type="calendar"
label="COM_CMGROUPBUYING_DEAL_FIELD_START_DATE_LABEL"
description="COM_CMGROUPBUYING_DEAL_FIELD_START_DATE_DESC"
class="inputbox"
size="22"
format="%Y-%m-%d %H:%M:%S"
required="true"
/>
<field
name="end_date"
type="calendar"
label="COM_CMGROUPBUYING_DEAL_FIELD_END_DATE_LABEL"
description="COM_CMGROUPBUYING_DEAL_FIELD_END_DATE_DESC"
class="inputbox"
size="22"
format="%Y-%m-%d %H:%M:%S"
required="true"
/>
with
<field
name="start_date"
type="calendar"
label="COM_CMGROUPBUYING_DEAL_FIELD_START_DATE_LABEL"
description="COM_CMGROUPBUYING_DEAL_FIELD_START_DATE_DESC"
class="inputbox"
size="22"
format="%Y-%m-%d %H:%M:%S"
required="true"
filter="server_utc" />
/>
<field
name="end_date"
type="calendar"
label="COM_CMGROUPBUYING_DEAL_FIELD_END_DATE_LABEL"
description="COM_CMGROUPBUYING_DEAL_FIELD_END_DATE_DESC"
class="inputbox"
size="22"
format="%Y-%m-%d %H:%M:%S"
required="true"
filter="server_utc" />
/>
And make sure you have already selected your time zone in Global Configuration.
Regards,
Hung