In ePO 4.6 in "Queries & Reports -> Shared Groups -> Product Deployments" there's already a pre-built query
Name: New Agents Added to ePO per Week
Notes: Great query during a rollout or tracking the number of new agents showing up in ePO on daily, weekly or monthly basis.
It even tells you who installed it.
Update: Opps sorry. missed where you said you had 4.5
Maybe if it's not in 4.5 (I don't have 4.5 to be able to check) you could build it manually
Basically it's a query against "Logging -> Audit Log Entries" with the criteria (Action Equals "New system" and Completion Time Is before now)