Please use milli-seconds as value to the attribute

URL:  <URL>/api/v3/worklog?OPERATION_NAME=add&TECHNICIAN_KEY=XXXXXXXXXXXXX

JSON: 

input_date={
"worklog": {
"request": {
"id": "6"
},
"description": "Adding a worklog",
"technician": {
"name": "administrator"
},
"total_time_spent": 720000,
"start_time": {
"value": 1421042555857
},
"end_time": {
"value": 1421042555857
}
}
}