A time capsule is sealed in a building or other site when a cache of interesting objects is buried, enclosed in a wall or cornerstone, or otherwise sealed away with the intention that a future generation will open it (usually at some designated date far in the future) to discover a snapshot of the historic moment when the capsule was sealed.

Time capsules are often marked by informative plaques indicating the date they were sealed as well as the date they are scheduled to be opened.

The cache is deliberately preserved for later discovery. Its sealing and later opening are separate events, and a scheduled opening date need not coincide with the eventual actual opening.