Add tasks for dates that have already passed.

[nus-se-bot]

An error message should occur since it does not logically make sense to add tasks for past dates.

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_{[original: nus-cs2103-AY2122S1/pe-interim#5762] [original labels: severity.Low type.FeatureFlaw]}

[ambroseboo]

Team's Response

Dates for tasks that have already passed are valid, since with the use of recurring frequency the dates can be set to past dates to be recurred in the future. Also, a past task can also be useful as a reminder to show that it is complete.

Duplicate status (if any):

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