Cement

The best hing to do is put the part bag in a bin bag and seal with good quality tape or tie up the end with string. Squeeze as much of the air out as possible It will store for months.

Left open it will go off in a fortnight or less, especially this humid weather.

a bit

Only open the cement bag enough to get what you want out, say a 4x4" flap so you can get cup size measure in. Put the whole bag in a bin bag and twist once and fold under the rest of the bag the open end, excluding as much air as possible. It'll keep for a long time but be aware that it might not be quite as good as fresh a year later... Keeping the damp out is what you need to do.

at that situation which begs the question...

same day so that it is not directly exposed to the air, does it last any longer?

Not worth the risk. Always throw it away.

Bill

Places such as Wickes sell cement in thick plastic bags. In my experience sealing these bags as you suggest enables the cement to be used long afterwards. It's probably more important to keep the moisture out and to get a bag without splits in the first place.

Having said this, if it was an "important" job I would buy fresh.

In what way does it "beg the question"?