convertCMC: Convert century month codes to Stata dates

CMCs (i.e. dates coded as months since the start of the year 1900), are widely used by the Demographic and Health Surveys program and others. convertCMC can randomly impute an exact day of the month to each date. This avoids (at least in aggregate) the biases that can arise when doing arithmetic with CMCs.

For example, a child that was born in January 2013, whose mother was interviewed in January 2018, may have been either 4 or 5 completed years old at that time depending on whether the mother was interviewed before or after the child's birthday. convertCMC takes into account the length of each month and knows about leap years.