Pralana Gold asks for my birthday on the Home page and then computes my age, but it's off by one year. It shows me being younger than I am. Why? Is this an error?
No, this is not an error. The age shown on the Home page is your age as of January 1 in the specified starting year, not your age on the current date. The tool works in increments of one year and when you look at its tabular projections you'll see one row for each year in the modeling period (time span of years from the starting year until your expected death year). Each of these rows has the year in column 1 and your age and your spouse's age at the start of that year in the next two columns. So, the age shown the Home page will correspond to the age(s) shown on the first row of the tool's projections.
Going further, if you have an income stream that begins on your birthday then you should see an indication of only a fraction of a year's worth of that income stream in the first year, where the fraction is a function of your birthday. Likewise, if you have an income stream that ends on your birthday then you should see an indication of only a fraction of a year's worth of that income stream in the last year.
