Merge Time into a DateTime to update Datetime
I need to update time info in a DateTime.

I get a string in the format "14" or "14:30" (for example), so I need to give it to Time parser to get the right hour. Then I need to update `self.start_at` which is a datetime which already has a time, but I need to update it.

    self.start_at_hours = Time.parse(self.start_at_hours) # example 14:30:00
    # NEED TO UPDATE self.start_at which is a datetime

I was using the `change` method on `self.start_at` but it only takes `hour` and `minutes` separated and I'm not sure what should I do.
Merge Time into a DateTime to update Datetime
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I need to update time info in a DateTime.

I get a string in the format "14" or "14:30" (for example), so I need to give it to Time parser to get the right hour. Then I need to update `self.start_at` which is a datetime which already has a time, but I need to update it.

    self.start_at_hours = Time.parse(self.start_at_hours) # example 14:30:00
    # NEED TO UPDATE self.start_at which is a datetime

I was using the `change` method on `self.start_at` but it only takes `hour` and `minutes` separated and I'm not sure what should I do.