You're dealing with default dates here, so to test for it, you need to use:
@if (item.JobEnded == default(DateTime))
{
@:In Progress
}
else
{
@Html.DisplayFor(m => item.JobEnded)
}
Or in one-liner form:
@((item.JobEnded == default(DateTime)) ? "In Progress" : Html.DisplayFor(m => item.JobEnded).ToString())