Question:

Below url is giving all the information except start and end dates of a sprint .

<a href="http://jira.example.com/rest/api/2/search?fields=assignee,key,summary,worklog,timeoriginalestimate,aggregatetimespent,&jql=project=BLAH+AND+sprint=57" rel="nofollow">http://jira.example.com/rest/api/2/search?fields=assignee,key,summary,worklog,timeoriginalestimate,aggregatetimespent,&jql=project=BLAH+AND+sprint=57</a>

Assuming you're a recent version of JIRA and JIRA Software, you can use the <a href="https://docs.atlassian.com/jira-software/REST/server/#agile/1.0" rel="nofollow">JIRA Agile REST API's</a>.

Interesting REST API resources are:

<a href="https://docs.atlassian.com/jira-software/REST/server/#agile/1.0/issue-getIssue" rel="nofollow">Get Issue</a>: GET /rest/agile/1.0/issue/{issueIdOrKey}<br /> The output of this resource will also include the agile fields and the sprint name and its id, e.g.:

... "sprint": { "id": 37, "self": "http://www.example.com/jira/rest/agile/1.0/sprint/13", "state": "future", "name": "sprint 2" }, ...

<a href="https://docs.atlassian.com/jira-software/REST/server/#agile/1.0/sprint-getSprint" rel="nofollow">Get Sprint</a>: GET /rest/agile/1.0/sprint/{sprintId} The output of this resource include the sprint start and end dates, e.g.:

{ "id": 37, "self": "http://www.example.com/jira/rest/agile/1.0/sprint/23", "state": "closed", "name": "sprint 1", "startDate": "2015-04-11T15:22:00.000+10:00", "endDate": "2015-04-20T01:22:00.000+10:00", "completeDate": "2015-04-20T11:04:00.000+10:00", "originBoardId": 5 }

The docs may also contain other useful resources for you.