He was promoted to Lieutenant on 15 August 1919, and in 1920 began survey work, serving on HMS Endeavour on the north coast of Egypt.

[8] In November 1944, Day took over command of HMS White Bear, a yacht converted for surveying, and carrying printing equipment and staff.

[3]: 174–175  After the Japanese surrender, White Bear conducted surveys around the Malay Peninsula and in Indonesia, to assist re-establishment of normal navigation.

[8] After the war, Day resumed his position as Assistant Hydrographer, and then in February 1948 took command of HMS Dalrymple, newly commissioned as a survey ship, working in the Mediterranean.

As Hydrographer, Day collaborated with the Decca Navigator Company to develop electronic systems for surveying using transportable shore stations.

[8] After retirement, Day led a small team to carry out a hydrographic survey of Lake Nyasa[11] In 1956 he was appointed coordinator of the International Geophysical Year, based in Brussels, but travelling extensively to facilitate the work of scientists from many countries that were taking part.