How many days are there in 70 years? How to calculate the total no. of days in 70 years (or any other no. of years) considering that this period also includes leap years?
It depends on which 70 years.  There might be anywhere from 16 through 18 leap years.  For example, the 70-year period from the beginning of 1901 through the end of 1970 contains 17 leap years (1904, 08, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48, 52, 56, 60, 64, 68), but the period from the beginning of 1904 through the end of 1973 contains 18 leap years (the same as the other, plus 1972).

To get 16 leap years, one observes that (for example) there was an exception in 1900, which was not a leap year, so that the period from 1897 through 1966 contains only 16 leap years. (1904 through 1964, the one in 1900 having been skipped.)

The solution to the general problem is something like this:  Let $Y$ be the number of years.  Calculate $Y\cdot 365$ and then add the leap days.  You can estimate the leap days by computing $Y\div 4$ and rounding off, but to get an exact count  the simplest thing to do is to check the years one at a time.  How you check the years depends on the calendar: in the Julian calendar, 1900 was a leap year, but in the Gregorian calendar, it was not.

In a different calendar, the answer will be completely different.  For example, in the Islamic calendar, the common year has not 365 but 355 days, and the leap year has 356 days; the Hebrew calendar is different again.
Actually, in the Gregorian calendar, it is a little more complicated than that!  For example, the year 1600, though divisible by 4, was NOT a leap year because it is also divisible by 400.  The year 2000, on the other hand, WAS a leap year because it was also divisible by 4000.  (It was the first time that particular provision was used.)