The age difference between two people is the span between their two dates of birth. Enter both dates and you see who is older and by exactly how much, in years, months, and days, and as a total number of days.

The gap is the calendar distance between the earlier and the later birth date, counted the same way as an age: full years, then whole months, then the days left over. The earlier date is the older person. Leap years and the different month lengths are handled exactly, so the answer is precise, not rounded to the nearest year.

• The gap between siblings, or between a parent and child.
• The difference between partners or friends.
• Any time you want the exact span between two birth dates instead of a rough 'a few years'.

People also use the gap for the old 'half your age plus seven' guideline. That's a rule of thumb, not a fact, but once you know the exact difference you can work it out for yourself. For one person, the same math gives your chronological age or your age in days, weeks, and months.