What happens if the government or your company declares an extra public holiday and it is not available in altHR? How can I add that into the system?

Worry not! Keep calm and go to Leaves > Settings > Holidays > Company Holidays > "Create Company Holiday"

By adding the holiday here, it will reflect in employees' leave schedules. 

However, please note that adding a holiday DOES NOT add the balance back into employees' leaves if they have applied for leave PRIOR to the changes. Please inform employees to withdraw or cancel their leave and resubmit for the changes to be reflected.