When Jesus laid aside His glory at the first Advent, He relinquished omniscience. So He could not tell the disciples when He would return. He didn't know! But He knew He would return. He knew the "would" but not the "when." We are in an identical situation.
Based on Matthew 24:29-44, this is the 3rd message in Stuart Briscoe's 4-message series, When Will Christ Return?