create tif (TimeIndexFrequency) codes for hourly, minutely, and
secondly ti's.
hourly(n = 0)
minutely(n = 0)
secondly(n = 0)number of base periods to skip. That is, hourly(2)
gives a tif code for a series observed every 2nd hour, while
both minutely() and minutely(1) are for a series
observed once per minute, secondly(30) means every 30 seconds,
and so on.
An integer tif code.
The current implementation has hourly(n) --> 2000 + n,
minutely(n) --> 3000 + n, and secondly(n) --> 4000 + n. If n
divides evenly into 3600 for secondly(n), the return code will
be the same as hourly(n/3600). For secondly(n) and
minutely(n), if n divides evenly into 60, the return
code will be as if minutely(n/60) or hourly(n/60) had
been called, respectively.
For hourly(n), n must evenly divide into 24 and be less
than 24, i.e., n is one of 1, 2, 3, 4, 6, 8, 12. For
minutely(n), n must be an even divisor of 1440, and less
than 720. For secondly(n), n must divide evenly into
86400, and be no larger than 960.