Cron task with scheduled timestamp in the past

I´m doing some setting to enable/disable a cron task and choose a time to run this task.

When I disable the cron task and enable it after without changed the time, my timestamp is in the past and make this error :

WP Control Plugin Result

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My code

If $cron_sync_time is a string “04:30”, why the timestamp should be in the past ?
Someone knows the way to fix this ?