input: Write a python function to execute a command based on voice input.
  assert execute_command('time') is not None
  assert execute_command('date') is not None
  assert execute_command('greeting') == "Hello, how can I help you?"
  assert 1 <= execute_command('random') <= 100
output: import datetime
  import random
  
  def execute_command(command):
      if command == 'time':
          return datetime.datetime.now().strftime("%H:%M:%S")
      elif command == 'date':
          return datetime.datetime.now().strftime("%Y-%m-%d")
      elif command == 'greeting':
          return "Hello, how can I help you?"
      elif command == 'random':
          return random.randint(1, 100)