The REST representation (and the internal APIs) are incorrect when the field used in the semantic are not populated in the artifact or when there is no semantic at all.
Depending on the case you can end up in a situation where the start date is set to 1970, some examples:
{
"field_id": 5019,
"type": "burndown",
"label": "burndown",
"value": {
"start_date": "1970-01-01T01:00:00+01:00",
"duration": 0,
"capacity": null,
"points": [],
"is_under_calculation": false,
"opening_days": [
1,
2,
3,
4,
5
],
"points_with_date": []
}
}
The root cause seems to be located in Tuleap\Tracker\Semantic\Timeframe\TimeframeBuilder::buildTimePeriodWithoutWeekendForArtifactForREST
which is also a source of issues for the ongoing story #18408.