The software functions only on complete 12-month time periods – in accordance with this assumption, note that everything is assumed to occur either at the very start of the year in question (in effect, 1st Jan), or at the very end (31st Dec). There is no concept of ‘pro-rating’, for part-years.
Regular contributions investments and savings are assumed to occur at the ‘end’ of the year in question, meaning that the contribution amount itself will not accrue growth in the year it is added to the account.
Likewise, withdrawals are assumed to occur at the end of the year in question.