Simple past tense and past participle of timestamp.(verb)
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Use "timestamped" in a sentence
"Each backup can be saved in a timestamped folder and you can set it to save x number of folders."
"Odd that this post is timestamped above at 12: 58pm, but just appeared after 6: 00pm (according to my Google Reader)."
"At 18: 57: 09 I received three additional transmissions: two duplicates of previous transmissions, and a new message, which is timestamped December 1st, 2185."