All three of these adverbs are considered acceptable for formal writing.
at an unspecified time in the future. : at an unspecified or unknown time in the
past. Source: Merriam-Webster's Learner's Dictionary. Examples:
The commonly confused words "
At an indefinite or unstated time: I'll meet you
“Let's get together
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Thousands of dollars were allegedly stolen from the bank's ATM vault
Dec 12, 2011