Correct logic for parsing and comparing sync time

[sabre1041]

Existing logic for determining when to prune groups was based on string comparison instead of proper datetime formatting. This enhancement corrects this logic.

Related to the following:

248

69

Mitigates the need for #251

[sabre1041]

@raffaelespazzoli Updated to align holistically on RFC3339

[raffaelespazzoli]

LGTM