Fig 1. North face of large cross at Sandbach. -----------Fig 2. Ladder calendar for 325 AD.

To calculate the date of construction

1. Assume that the scale is linear arround the time of the equinox ( near the end of March).

2. Using the step between the 1st of April and the 1st of September. The number of days is given by

30 x 5.25 / 20.5 = 7.68 days

3. From the ladder calendar for the year 325 AD. the corresponding number of days from the 1st of April to the 1st of September is 12 days.

4. Therefore the calendar has slipped by :- ( 12 - 7.68 ) / 2 = 2.16 days.

5. The Julian calendar was adding an extra day every 128 years. So a slipage of 2.16 days is equal to a time of 128 x 2.16 = 276 years.

6. Then the date of construction = 276 + 325 = 601 AD.