Schedule Meeting time accepts any string. Needs time validation

Currently the bot with either Facebook or Slack will accept any string for time and schedule it. 

User story:
user: setup meeting
bot: At what time (ex. 15:30) do you want me to set up the meeting?
user: no
bot: you meeting is set at no. Would you like to repeat it tomorrow?

Some sort of time validation should be implemented to avoid this bug.