Why does SyncTime not use hashes to compare files?

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SyncTime offers the option to compare the modification date and/or size of a file to avoid comparing the actual file contents, which depending on the file size could take a long time.

A hash is a short character sequence computed from a given input (e.g. a file). A hash is, with high probability, unique for different inputs, and can hence be used to compare the inputs.

Hashes would be convenient to compare files if they would readily be available. In order to compute the hash of a file, the whole file contents need to be read first. So computing hashes of the files to compare is usually at least as expensive as comparing the file contents directly, or even just copying the source to the destination.