You don’t need sunscreen when you’re going to work in the early morning or returning home after 6pm, because the UV rays are weaker at those times. But between March and October, a good sunscreen of at least SPF30, should always be used between 11am and 3pm, when the sun is at its highest.
This applies even if you are just popping to the shops or hanging out the washing in the garden, because the risk starts the moment you step outside. Two minutes spent outside may produce only a little damage, but all these little exposures add up over a lifetime.
Another point to remember is that clouds do not absorb UV very well, and neither does rain. If the cloud is low it will reduce the UV, but you’d still burn – it would just take about twice as long, so rain or shine, be sure to apply a good sunscreen. Don’t leave your sunscreen at home for your aesthetic beauty!