Delphi - restar horas

como puedo construir una formula, que me permite restar dos horas, hora inicial y la actual, y que sin importar que la hora actual represente otro dia diferente de la hora inicial, puede darme el numero de horas transcurridas entre esas horas.

por ejemplo.
fecha de entrada 2 de mayo hora_e :=10:30 p.m + limite:=4
al comparar la hora actual
si son las 2:00 a.m 3 de mayo
al comparar esas horas me puede darme el tiempo transcurrido.

Hola:

Mirate las funciones HoursBetween y HourSpan de la unit DateUtils.

function HoursBetween(const ANow, AThen: TDateTime): Int64;

Call HoursBetween to obtain the difference, in hours, between two TDateTime values. HoursBetween counts only entire hours. Thus, HoursBetween reports the difference between 9:00 AM and 9:59:59 AM as 0 because the difference is one second short of an entire hour.

function HourSpan(const ANow, AThen: TDateTime): Double;

Call HourSpan to obtain the difference, in hours, between two TDateTime values. Unlike the HoursBetween function, which only counts entire hours, HourSpan reports incomplete hours as a fraction of an entire hour.

Suerte. Virum.