Get last modified date using GitHub GraphQL API

Query the first item in the history of that path and return the committedDate:

query CommittedDate($name: String!, $owner: String!, $path: String!) {
repository(owner: $owner, name: $name) {
ref(qualifiedName: "refs/heads/master") {
target {
... on Commit {
history(first: 1, path: $path) {
edges {
node {
committedDate
}
}
}
}
}
}
}
}