diff --git a/routers/api/v1/repo/issue.go b/routers/api/v1/repo/issue.go
index c3d9dfac1d8..a8a6bb1ddf2 100644
--- a/routers/api/v1/repo/issue.go
+++ b/routers/api/v1/repo/issue.go
@@ -22,7 +22,6 @@ import (
 	"gitea.dev/modules/optional"
 	"gitea.dev/modules/setting"
 	api "gitea.dev/modules/structs"
-	"gitea.dev/modules/timeutil"
 	"gitea.dev/modules/util"
 	"gitea.dev/modules/web"
 	"gitea.dev/routers/api/v1/utils"
@@ -568,25 +567,20 @@ func CreateIssue(ctx *context.APIContext) {
 	//     "$ref": "#/responses/repoArchivedError"
 
 	form := web.GetForm[*api.CreateIssueOption](ctx)
-	var deadlineUnix timeutil.TimeStamp
-	if form.Deadline != nil && ctx.Repo.Permission.CanWrite(unit.TypeIssues) {
-		deadlineUnix = timeutil.TimeStamp(form.Deadline.Unix())
-	}
-
 	issue := &issues_model.Issue{
-		RepoID:       ctx.Repo.Repository.ID,
-		Repo:         ctx.Repo.Repository,
-		Title:        form.Title,
-		PosterID:     ctx.Doer.ID,
-		Poster:       ctx.Doer,
-		Content:      form.Body,
-		Ref:          form.Ref,
-		DeadlineUnix: deadlineUnix,
+		RepoID:   ctx.Repo.Repository.ID,
+		Repo:     ctx.Repo.Repository,
+		Title:    form.Title,
+		PosterID: ctx.Doer.ID,
+		Poster:   ctx.Doer,
+		Content:  form.Body,
+		Ref:      form.Ref,
 	}
 
 	assigneeIDs := make([]int64, 0)
 	var err error
 	if ctx.Repo.Permission.CanWrite(unit.TypeIssues) {
+		issue.DeadlineUnix = common.ParseAPIDeadlineToEndOfDay(form.Deadline)
 		issue.MilestoneID = form.Milestone
 		assigneeIDs, err = issues_model.MakeIDsFromAPIAssigneesToAdd(ctx, form.Assignee, form.Assignees)
 		if err != nil {
@@ -760,26 +754,8 @@ func EditIssue(ctx *context.APIContext) {
 	}
 
 	// Update or remove the deadline, only if set and allowed
-	if (form.Deadline != nil || form.RemoveDeadline != nil) && canWrite {
-		var deadlineUnix timeutil.TimeStamp
-
-		if form.RemoveDeadline == nil || !*form.RemoveDeadline {
-			if form.Deadline == nil {
-				ctx.APIError(http.StatusBadRequest, "The due_date cannot be empty")
-				return
-			}
-			if !form.Deadline.IsZero() {
-				deadline := time.Date(form.Deadline.Year(), form.Deadline.Month(), form.Deadline.Day(),
-					23, 59, 59, 0, form.Deadline.Location())
-				deadlineUnix = timeutil.TimeStamp(deadline.Unix())
-			}
-		}
-
-		if err := issues_model.UpdateIssueDeadline(ctx, issue, deadlineUnix, ctx.Doer); err != nil {
-			ctx.APIErrorInternal(err)
-			return
-		}
-		issue.DeadlineUnix = deadlineUnix
+	if canWrite && !editIssueDeadline(ctx, issue, form.Deadline, form.RemoveDeadline) {
+		return
 	}
 
 	// Add/delete assignees
@@ -911,6 +887,25 @@ func DeleteIssue(ctx *context.APIContext) {
 	ctx.Status(http.StatusNoContent)
 }
 
+func editIssueDeadline(ctx *context.APIContext, issue *issues_model.Issue, deadline *time.Time, removeDeadline *bool) bool {
+	if deadline == nil && removeDeadline == nil {
+		return true
+	}
+	if removeDeadline != nil && *removeDeadline {
+		deadline = nil
+	} else if deadline == nil {
+		ctx.APIError(http.StatusBadRequest, "The due_date cannot be empty")
+		return false
+	}
+	deadlineUnix := common.ParseAPIDeadlineToEndOfDay(deadline)
+	if err := issues_model.UpdateIssueDeadline(ctx, issue, deadlineUnix, ctx.Doer); err != nil {
+		ctx.APIErrorInternal(err)
+		return false
+	}
+	issue.DeadlineUnix = deadlineUnix
+	return true
+}
+
 // UpdateIssueDeadline updates an issue deadline
 func UpdateIssueDeadline(ctx *context.APIContext) {
 	// swagger:operation POST /repos/{owner}/{repo}/issues/{index}/deadline issue issueEditIssueDeadline
@@ -964,7 +959,7 @@ func UpdateIssueDeadline(ctx *context.APIContext) {
 		return
 	}
 
-	deadlineUnix, _ := common.ParseAPIDeadlineToEndOfDay(form.Deadline)
+	deadlineUnix := common.ParseAPIDeadlineToEndOfDay(form.Deadline)
 	if err := issues_model.UpdateIssueDeadline(ctx, issue, deadlineUnix, ctx.Doer); err != nil {
 		ctx.APIErrorInternal(err)
 		return
diff --git a/routers/api/v1/repo/milestone.go b/routers/api/v1/repo/milestone.go
index 2b19d43b889..41a7361e3ca 100644
--- a/routers/api/v1/repo/milestone.go
+++ b/routers/api/v1/repo/milestone.go
@@ -149,16 +149,11 @@ func CreateMilestone(ctx *context.APIContext) {
 	//     "$ref": "#/responses/notFound"
 	form := web.GetForm[*api.CreateMilestoneOption](ctx)
 
-	var deadlineUnix int64
-	if form.Deadline != nil {
-		deadlineUnix = form.Deadline.Unix()
-	}
-
 	milestone := &issues_model.Milestone{
 		RepoID:       ctx.Repo.Repository.ID,
 		Name:         form.Title,
 		Content:      form.Description,
-		DeadlineUnix: timeutil.TimeStamp(deadlineUnix),
+		DeadlineUnix: common.ParseAPIDeadlineToEndOfDay(form.Deadline),
 	}
 
 	if form.State == "closed" {
@@ -219,7 +214,7 @@ func EditMilestone(ctx *context.APIContext) {
 	if form.Description != nil {
 		milestone.Content = *form.Description
 	}
-	milestone.DeadlineUnix, _ = common.ParseAPIDeadlineToEndOfDay(form.Deadline)
+	milestone.DeadlineUnix = common.ParseAPIDeadlineToEndOfDay(form.Deadline)
 
 	oldIsClosed := milestone.IsClosed
 	if form.State != nil {
diff --git a/routers/api/v1/repo/pull.go b/routers/api/v1/repo/pull.go
index 0063287f77f..4cda175700d 100644
--- a/routers/api/v1/repo/pull.go
+++ b/routers/api/v1/repo/pull.go
@@ -10,7 +10,6 @@ import (
 	"net/http"
 	"strconv"
 	"strings"
-	"time"
 
 	git_model "gitea.dev/models/git"
 	issues_model "gitea.dev/models/issues"
@@ -26,7 +25,6 @@ import (
 	"gitea.dev/modules/optional"
 	"gitea.dev/modules/setting"
 	api "gitea.dev/modules/structs"
-	"gitea.dev/modules/timeutil"
 	"gitea.dev/modules/util"
 	"gitea.dev/modules/web"
 	"gitea.dev/routers/api/v1/utils"
@@ -491,11 +489,6 @@ func CreatePullRequest(ctx *context.APIContext) {
 		milestoneID = milestone.ID
 	}
 
-	var deadlineUnix timeutil.TimeStamp
-	if form.Deadline != nil {
-		deadlineUnix = timeutil.TimeStamp(form.Deadline.Unix())
-	}
-
 	unitPullRequest, err := ctx.Repo.Repository.GetUnit(ctx, unit.TypePullRequests)
 	if err != nil {
 		ctx.APIErrorInternal(err)
@@ -510,7 +503,7 @@ func CreatePullRequest(ctx *context.APIContext) {
 		MilestoneID:  milestoneID,
 		IsPull:       true,
 		Content:      form.Body,
-		DeadlineUnix: deadlineUnix,
+		DeadlineUnix: common.ParseAPIDeadlineToEndOfDay(form.Deadline),
 	}
 	pr := &issues_model.PullRequest{
 		HeadRepoID: compareResult.HeadRepo.ID,
@@ -616,6 +609,8 @@ func EditPullRequest(ctx *context.APIContext) {
 	// responses:
 	//   "201":
 	//     "$ref": "#/responses/PullRequest"
+	//   "400":
+	//     "$ref": "#/responses/error"
 	//   "403":
 	//     "$ref": "#/responses/forbidden"
 	//   "404":
@@ -690,19 +685,8 @@ func EditPullRequest(ctx *context.APIContext) {
 	}
 
 	// Update or remove deadline if set
-	if form.Deadline != nil || form.RemoveDeadline != nil {
-		var deadlineUnix timeutil.TimeStamp
-		if (form.RemoveDeadline == nil || !*form.RemoveDeadline) && !form.Deadline.IsZero() {
-			deadline := time.Date(form.Deadline.Year(), form.Deadline.Month(), form.Deadline.Day(),
-				23, 59, 59, 0, form.Deadline.Location())
-			deadlineUnix = timeutil.TimeStamp(deadline.Unix())
-		}
-
-		if err := issues_model.UpdateIssueDeadline(ctx, issue, deadlineUnix, ctx.Doer); err != nil {
-			ctx.APIErrorInternal(err)
-			return
-		}
-		issue.DeadlineUnix = deadlineUnix
+	if !editIssueDeadline(ctx, issue, form.Deadline, form.RemoveDeadline) {
+		return
 	}
 
 	// Add/delete assignees
diff --git a/routers/common/deadline.go b/routers/common/deadline.go
index b8a1449a687..43b30462835 100644
--- a/routers/common/deadline.go
+++ b/routers/common/deadline.go
@@ -22,10 +22,9 @@ func ParseDeadlineDateToEndOfDay(date string) (timeutil.TimeStamp, error) {
 	return timeutil.TimeStamp(deadline.Unix()), nil
 }
 
-func ParseAPIDeadlineToEndOfDay(t *time.Time) (timeutil.TimeStamp, error) {
+func ParseAPIDeadlineToEndOfDay(t *time.Time) timeutil.TimeStamp {
 	if t == nil || t.IsZero() || t.Unix() == 0 {
-		return 0, nil
+		return 0
 	}
-	deadline := time.Date(t.Year(), t.Month(), t.Day(), 23, 59, 59, 0, setting.DefaultUILocation)
-	return timeutil.TimeStamp(deadline.Unix()), nil
+	return timeutil.TimeStamp(time.Date(t.Year(), t.Month(), t.Day(), 23, 59, 59, 0, setting.DefaultUILocation).Unix())
 }
diff --git a/templates/swagger/v1-openapi3.generated.json b/templates/swagger/v1-openapi3.generated.json
index b92a5a134c6..46fcfccaeab 100644
--- a/templates/swagger/v1-openapi3.generated.json
+++ b/templates/swagger/v1-openapi3.generated.json
@@ -29212,6 +29212,9 @@
           "201": {
             "$ref": "#/components/responses/PullRequest"
           },
+          "400": {
+            "$ref": "#/components/responses/error"
+          },
           "403": {
             "$ref": "#/components/responses/forbidden"
           },
diff --git a/templates/swagger/v1-swagger.generated.json b/templates/swagger/v1-swagger.generated.json
index 7ef18edf193..7ffc1ebd418 100644
--- a/templates/swagger/v1-swagger.generated.json
+++ b/templates/swagger/v1-swagger.generated.json
@@ -16853,6 +16853,9 @@
           "201": {
             "$ref": "#/responses/PullRequest"
           },
+          "400": {
+            "$ref": "#/responses/error"
+          },
           "403": {
             "$ref": "#/responses/forbidden"
           },
diff --git a/tests/integration/api_issue_test.go b/tests/integration/api_issue_test.go
index b77775d1730..69c5c412cfe 100644
--- a/tests/integration/api_issue_test.go
+++ b/tests/integration/api_issue_test.go
@@ -126,11 +126,13 @@ func testAPICreateIssue(t *testing.T) {
 		Body:     body,
 		Title:    title,
 		Assignee: owner.Name,
+		Deadline: new(time.Date(2026, 5, 11, 12, 0, 0, 0, time.FixedZone("", -10*3600))),
 	}).AddTokenAuth(token)
 	resp := MakeRequest(t, req, http.StatusCreated)
 	apiIssue := DecodeJSON(t, resp, &api.Issue{})
 	assert.Equal(t, body, apiIssue.Body)
 	assert.Equal(t, title, apiIssue.Title)
+	assert.Equal(t, time.Date(2026, 5, 11, 23, 59, 59, 0, setting.DefaultUILocation).Unix(), apiIssue.Deadline.Unix())
 
 	unittest.AssertExistsAndLoadBean(t, &issues_model.Issue{
 		RepoID:     repoBefore.ID,
diff --git a/tests/integration/api_pull_test.go b/tests/integration/api_pull_test.go
index e9ccbe45831..aa4aed97895 100644
--- a/tests/integration/api_pull_test.go
+++ b/tests/integration/api_pull_test.go
@@ -449,13 +449,15 @@ func TestAPIEditPull(t *testing.T) {
 	token := getTokenForLoggedInUser(t, session, auth_model.AccessTokenScopeWriteRepository)
 	title := "create a success pr"
 	req := NewRequestWithJSON(t, http.MethodPost, fmt.Sprintf("/api/v1/repos/%s/%s/pulls", owner10.Name, repo10.Name), &api.CreatePullRequestOption{
-		Head:  "develop",
-		Base:  "master",
-		Title: title,
+		Head:     "develop",
+		Base:     "master",
+		Title:    title,
+		Deadline: new(time.Date(2026, 5, 11, 12, 0, 0, 0, time.FixedZone("", -10*3600))),
 	}).AddTokenAuth(token)
 	resp := MakeRequest(t, req, http.StatusCreated)
 	apiPull := DecodeJSON(t, resp, &api.PullRequest{})
 	assert.Equal(t, "master", apiPull.Base.Name)
+	assert.Equal(t, time.Date(2026, 5, 11, 23, 59, 59, 0, setting.DefaultUILocation).Unix(), apiPull.Deadline.Unix())
 
 	newTitle := "edit a this pr"
 	newBody := "edited body"
@@ -480,6 +482,11 @@ func TestAPIEditPull(t *testing.T) {
 	}).AddTokenAuth(token)
 	MakeRequest(t, req, http.StatusNotFound)
 
+	req = NewRequestWithJSON(t, http.MethodPatch, fmt.Sprintf("/api/v1/repos/%s/%s/pulls/%d", owner10.Name, repo10.Name, pull.Index), &api.EditPullRequestOption{
+		RemoveDeadline: new(false),
+	}).AddTokenAuth(token)
+	MakeRequest(t, req, http.StatusBadRequest)
+
 	t.Run("PullContentVersion", func(t *testing.T) {
 		testAPIPullContentVersion(t, pull.ID)
 	})