Philadelphia is typically 5 hours behind London. When it's 3:00 PM in London, it's 10:00 AM in Philadelphia. Both cities observe daylight saving time, but the switch dates differ slightly between the US and UK.
During the brief periods when the US and UK change clocks on different weekends (usually in March and November), the difference can temporarily be 4 or 6 hours. For most of the year, it's a consistent 5-hour difference.
Optimal meeting window: 8:00 AM - 11:00 AM Philadelphia time (1:00 PM - 4:00 PM London). This catches afternoon business hours in London while being morning in Philadelphia.
Pharmaceutical and healthcare companies in both cities frequently use this window for coordination.