diff --git a/src/handlers/annotations.go b/src/handlers/annotations.go
index b1570ce..87b8838 100644
--- a/src/handlers/annotations.go
+++ b/src/handlers/annotations.go
@@ -1,6 +1,7 @@
 package handlers
 
 import (
+	"fmt"
 	"net/http"
 	"sheetless-server/database"
 	"sheetless-server/models"
@@ -17,6 +18,27 @@ type AnnotationRequest struct {
 	Annotations  string `json:"annotations" binding:"required"`
 }
 
+// parseFlexibleTime parses time strings in various ISO8601/RFC3339 formats
+func parseFlexibleTime(s string) (time.Time, error) {
+	// Try multiple formats that Dart/Flutter might send
+	formats := []string{
+		time.RFC3339,
+		time.RFC3339Nano,
+		"2006-01-02T15:04:05.999999999",  // Dart ISO8601 without timezone
+		"2006-01-02T15:04:05.999999",     // Dart ISO8601 with microseconds
+		"2006-01-02T15:04:05.999",        // Dart ISO8601 with milliseconds
+		"2006-01-02T15:04:05",            // Dart ISO8601 without fractional seconds
+	}
+
+	for _, format := range formats {
+		if t, err := time.Parse(format, s); err == nil {
+			return t, nil
+		}
+	}
+
+	return time.Time{}, fmt.Errorf("unable to parse time: %s", s)
+}
+
 // GetAnnotations returns all annotations for a sheet
 // GET /api/sheets/:uuid/annotations
 func GetAnnotations(c *gin.Context) {
@@ -65,10 +87,10 @@ func UpdateAnnotation(c *gin.Context) {
 		return
 	}
 
-	// Parse lastModified timestamp
-	lastModified, err := time.Parse(time.RFC3339, req.LastModified)
+	// Parse lastModified timestamp (flexible format to support Dart/Flutter)
+	lastModified, err := parseFlexibleTime(req.LastModified)
 	if err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid lastModified format, expected RFC3339"})
+		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid lastModified format: " + err.Error()})
 		return
 	}
 
diff --git a/src/handlers/changes.go b/src/handlers/changes.go
index f62ccef..430015c 100644
--- a/src/handlers/changes.go
+++ b/src/handlers/changes.go
@@ -4,7 +4,6 @@ import (
 	"net/http"
 	"sheetless-server/database"
 	"sheetless-server/models"
-	"time"
 
 	"github.com/gin-gonic/gin"
 	"github.com/google/uuid"
@@ -59,8 +58,8 @@ func SyncChanges(c *gin.Context) {
 			continue // Skip invalid UUIDs
 		}
 
-		// Parse createdAt timestamp
-		createdAt, err := time.Parse(time.RFC3339, change.CreatedAt)
+		// Parse createdAt timestamp (flexible format to support Dart/Flutter)
+		createdAt, err := parseFlexibleTime(change.CreatedAt)
 		if err != nil {
 			continue // Skip invalid timestamps
 		}