If you really want to put delta-T in a program then there are polynomial approximations such as the ones here. As has been said though you are unlikely to require this for lunar phases. It is important for high precision ephemerides of relatively close objects (including satellites) since it does affect the parallax correction and, as Dominic says, it is crucial for calculating total eclipse paths. It is probably worth including in telescope pointing algorithms too. Should anyone ever decide to remove leap seconds from UTC it will become important for the general public in a few thousand years!