Om—this syllable is all this. Its explanation: all that is past, present, and future is simply Om; and whatever else is beyond the three times, that too is simply Om.
ॐ इत्येतदक्षरमिदꣳ सर्वं तस्योपव्याख्यानं भूतं भवद् भविष्यदिति सर्वमोङ्कार एव यच्चान्यत् त्रिकालातीतं तदप्योङ्कार एव ॥ १॥