I will agree with Khalid on that one.
A simple wall clock and a cheap DS1307/DS1337/DS3231 will keep really accurate time over half a year span with no problems. How long are your installation going to run ? Anyway, the RTC will be way more accurate than the mechanical clock, so once you set the RTC, you won't need to worry about that anymore, just adjust the mechanical clock to accurate time once in a while, if you are not satisfied with it's accuracy.
// Per.