Exodus 12:6 and 12:18 appear to present a conflict when viewed through the sunrise reckoning of the day. These verses point to a specific point in time and at specific events.
Both points in time occur on the fourteenth day of the month, at even, or between the evenings. In the sunrise reckoning of the day, there is only one point in time that these verses can occur. That point in time is at the end of the daylight period of the 14th day, beginning the evening and night period.
I believe the difficulty in reconciling this scenario is caused by a misunderstanding or lack understanding of the phrase "between the evenings." This phrase occurs about half a dozen times in OT scripture at very key points of understanding. If you do not have an understanding of "between the evenings," this may be where you need to begin examining this dilemma.
If you believe this challenge to be in error, please feel free to challenge this material. I remain open to learning if this conflict can be resolved by scripture.