This is a bit more difference than I would like and the reasons for it are several, for example whether seeds are started indoors or sown direct, whether plants are fleeced for a fortnight after planting, what the climate is (last year the pea harvest was ten days later than in 2011, sown at the same time) and what stage of maturity you like for a first harvest (petit pois gain a week over full sized peas), and whether seeds are sown now or in late April.
In the book’s small tables there is not room to cover every eventuality so my figure is to give an idea of what is possible. Your mother in law may desire a cold spring to give her your harvest! I would sow undercover in a week or so and you may hope for most of the harvest before you go, but there are few certainties in gardening.