Allow for negative Start Date

[JonasunderscoreJones]

Allow for negative Offset to allow Starting a Month in the previous Month.

Example (useful for people who live from paycheck to paycheck):

• Income arrives on the 27th of March but is being spent in April.
• The interval of the Month April then goes from the 27th of March to the 26th of April
• Currently only a positive Offset is allowed so if You enter 27 as the offset, the Month of April goes from the 27th of April to the 27th of May which is exactly one Month too late