GNU Wget 中的时间戳是 使用“--timestamping”打开 ('-N') 选项，或通过时间戳 = on .wgetrc 中的指令。使用此选项，对于每个文件，它打算 下载后，Wget 将检查是否有 本地存在同名文件。如果 确实如此，但远程文件不是 较新的，Wget 将不会下载它。
如果本地文件不存在，或者 文件大小不匹配， wget 将下载远程文件 no 不管时间戳怎么说。
Take a look at the timestamping section in the wget manual:
Time-Stamping
One of the most important aspects of mirroring information from the Internet is updating your archives.
Downloading the whole archive again and again, just to replace a few changed files is expensive, both in terms of wasted bandwidth and money, and the time to do the update. This is why all the mirroring tools offer the option of incremental updating.
Such an updating mechanism means that the remote server is scanned in search of new files. Only those new files will be downloaded in the place of the old ones.
A file is considered new if one of these two conditions are met:
A file of that name does not already exist locally.
A file of that name does exist, but the remote file was modified more recently than the local file.
To implement this, the program needs to be aware of the time of last modification of both local and remote files. We call this information the time-stamp of a file.
The time-stamping in GNU Wget is turned on using ‘--timestamping’ (‘-N’) option, or through timestamping = on directive in .wgetrc. With this option, for each file it intends to download, Wget will check whether a local file of the same name exists. If it does, and the remote file is not newer, Wget will not download it.
If the local file does not exist, or the sizes of the files do not match, Wget will download the remote file no matter what the time-stamps say.
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查看 wget 手册中的 时间戳 部分：
Take a look at the timestamping section in the wget manual:
wget -N http://server/path/to/file.txtwget -N http://server/path/to/file.txt