Fix the birthdays calendar by refactoring the start/end date query parsing

Ghost User
2018-11-08T14:45:26+01:00

Closes #764

#### Previous behaviour
Steps to reproduce:

1. No birthdays in calendar

#### New behaviour
Steps to validate that it works:

1. Birthdays in calendar work

Closes #764 (closed)

Previous behaviour

Steps to reproduce:

1. No birthdays in calendar

New behaviour

Steps to validate that it works:

1. Birthdays in calendar work