Initial implementation

internal/models/diary.go

@@ -0,0 +1,14 @@
+package models
+
+import "time"
+
+// DiaryEntry represents a journal entry for a specific day.
+type DiaryEntry struct {
+	Date time.Time
+	Text string
+}
+
+// DateString returns the date formatted as YYYY-MM-DD.
+func (d *DiaryEntry) DateString() string {
+	return d.Date.Format("2006-01-02")
+}