Following an 8-month time/frequency (T/F) incubator project, this report sets out recommendations for building a GÉANT Core Time/Frequency Network (C-TFN) comprising federated cross-border T/F links that interconnect existing national T/F networks. The report includes the proposed architecture of the GÉANT C-TFN, topology options, cost modelling, sustainability, funding, operation and maintenance, recommendations for GN5-2, and areas requiring further investigation.