fix: use system timezone in gr.DateTime with include_time=False

[lcian]

Description

When using gr.DateTime with include_time=False, the frontend code creates a new Date object out of the chosen date string.

There is a discrepancy in the way date-only and date-time strings are dealt with in JS by the Date constructor, which causes the date string to be interpreted as if it specified a time in the UTC timezone (source):

When the time zone offset is absent, date-only forms are interpreted as a UTC time and date-time forms are interpreted as a local time. The interpretation as a UTC time is due to a historical spec error that was not consistent with ISO 8601 but could not be changed due to web compatibility.

Due to this behavior, if the user's system timezone is one which has a negative offset with respect to UTC, the date returned by a gr.DateTime component with include_time=False would correspond to the day preceding the one selected by the user, which is wrong.

By appending "T00:00:00", we turn the date-only string into a date-time string, which will be parsed correctly as per the user's system timezone (not necessarily UTC).

Closes: #9573

[gradio-pr-bot]

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gradio	patch

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fix: use system timezone in gr.DateTime with include_time=False

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[aliabid94]

Beautiful, thanks!