Yes, It is Time Zone issue only. In your post request, you need to convert date to GMT+00:00 format and then save it to salesforce.

    DateTime departureDateTimeGMTFormat = Datetime.newInstanceGmt( DeptDate.year(), DeptDate.month(), DeptDate.day(), integer.Valueof(DeptTime.substring(0, 2)),integer.Valueof( DeptTime.substring(2, 4)),00 );
    
    DeptDateTime = departureDateTimeGMTFormat.format( 'yyyy.MM.dd HH:mm' , 'GMT+5:30');