If I do the PCA on the whole dataset I get 7 components that can explain 90% of the variance, if I split the dataset into 2 (sorted by time), the number of significant components in the first half goes to 5 (with 15 variables present in one or more components) and in the second half goes to 8 (with 21 variables present in one or more components), can we infer that some of these variables become more significant in the latter half compared to first half?
I would say that it depends on your data. If it is some sample (which I am sure it is), then you need to be careful about such inferences, since the first half your observations might not have been representative of your underlying population, and of course the same can be said about the latter half. So much in general about variable importance.
As far as 'becoming more important over time' is concerned, I have not heard about analyzing the change of variable importance in PCA (though I no PCA expert!). In either case, I would imagine that you would probably have to account for the importance of the variable within the component, as well as for the component loading itself. Assuming that your sample data does represent the reality reasonably well, you could probably make statements about it, but unless you find some well documented method that mathematically proves that it means anything, it is vague at best.
In your case, I would rather cross-validate the PCA in both datasets and check whether the differences are not purely random.