Update comments

Author: harryzcy

services/migrations/github.go

@@ -801,6 +801,7 @@ func (g GithubDownloaderV3) GetAllNewComments(page, perPage int, updatedAfter ti
 }
 
 // GetNewPullRequests returns pull requests after the given time according page and perPage
+// If `updatedAfter` is zero-valued, it will return all pull requests
 func (g *GithubDownloaderV3) GetNewPullRequests(page, perPage int, updatedAfter time.Time) ([]*base.PullRequest, bool, error) {
 	// Pulls API doesn't have parameter `since`, so we have to use Search API instead.
 	// By specifying `repo:owner/repo is:pr` in the query, we can get all pull requests of the repository.
@@ -823,7 +824,8 @@ func (g *GithubDownloaderV3) GetNewPullRequests(page, perPage int, updatedAfter
 
 	searchQuery := fmt.Sprintf("repo:%s/%s is:pr", g.repoOwner, g.repoName)
 	if !updatedAfter.IsZero() {
-		// timezone denoted by plus, rather than 'Z',
+		// GitHub requires time to be later than 1970-01-01, so we should skip `updated` part if it's zero.
+		// Timezone is denoted by plus/minus UTC offset, rather than 'Z',
 		// according to https://docs.github.com/en/search-github/searching-on-github/searching-issues-and-pull-requests#search-by-when-an-issue-or-pull-request-was-created-or-last-updated
 		timeStr := updatedAfter.Format("2006-01-02T15:04:05-07:00")
 		searchQuery += fmt.Sprintf(" updated:>=%s", timeStr)