What is aparaahnam? It is said that tharppanam must be performed only during aparaahnam. Adiyen usually does the tharppanam after 1 PM. Kindly clarify whether this is appropriate.

For a given day, the daytime is split into 5 periods. The 4th period is called Aparaahnam. It is the Afternoon in English.
A daytime exists approximately for 30 Nazhigais. Dividing this into 5 parts, the duration of each part is 6 Nazhigais. The period between 18th Nazhigai and 24th Nazhigai is called aparaahnam. It occurs roughly after 1 PM. Hence, performing tharppanam is appropriate.