Electronic Health Records (EHRs) are widely used in the United States for clinical care and billing activities. Their widespread adoption has raised a variety of concerns about their effects on providers and medical care. As researchers address these concerns, they will need to understand how much time providers actually spend on the EHR. This study develops and validates methods for calculating total time requirements for EHR use by ophthalmologists using secondary EHR data from audit logs. Key findings from this study are that (1) Secondary EHR data can be used to estimate lower bounds on provider EHR use, (2) Providers spend a large amount of time using the EHR, (3) Most time spent on the EHR is spent reviewing information. These findings have important implications for practicing clinicians, and for EHR system design in the future.