1 : Repousse ton voyage, ne serait-ce que d’un jour !
{au lieu de} : Repousse ton voyage, ne serait-ce que d’une journée !
2 : Que la vie subsiste, ne serait-ce qu’une journée de plus.
{au lieu de} : Que la vie subsiste, ne serait-ce qu’un jour de plus.
In most cases, I’m familiar with the difference of usage between “un jour” and “une journée”, but not in this particular instance.
When you're counting days purely as a countdown or a duration, without caring about what you do during them, it's always "jour". You say "dans trois jours", but never "dans trois journées". So when you're in a idea of counting days but it happens to be one (like counting by how many days you'll postpone something), use "jour"
When you use "journée", even in plural (you can!), you're often highlighting what you do during them : "il passe ses journées à rien faire". So it makes sense, when talking about living the present day, to use "journée" instead of "jour". (Notice you're not counting them precisely)
However, in the second example, it's not mandatory to use "journée", "jour" would have worked fine, you're also kind of counting days.
There are some cases where the two are interchangeable, sometimes it sounds more poetic to use one or the other.
so basically, if you stay "one more day", you can use both of them. The idea is that you have to enjoy your stay so during the day, you will generally not enjoy your night (but you can too of course).
Both sentences have the same meaning here.
