This method will use an estimate if USE_ESTIMATED_UTCNOW is 1. To calculate the estimate it will initialize a base reference time and keep track of the offset from that time with FPlatformTime::Cycles64(). This is appropriate for platforms whose implementations of FPlatformTime::UtcTime are expensive. To use, enable bUseEstimatedUtcNow in TargetRules. The rebase time is stored in time.EstimatedUtcNowRebaseTimeSeconds (default setting is 600 seconds). Current date and time.