When importing Executions Results, it is possible to set the date values like Begin Date or End Date using a multipart endpoint.
In the multipart endpoint, it sends a second file in JSON format with the additional fields and respective values to set up.
And the format for date values is the ISO 8601 (YYYY-MM-DDThh:mm:ss.sTZD) like shown in the example below:
{ "fields": { "customfield_10003": "2020-03-19T10:29:29.908+1100" } }
For additional information on all the available multipart endpoints, please check Import Execution Results in our documentation.