A tetrad is one of four parts of a month. Since most months are not a multiple of a week long (except for February in non-leap years), tetrads do not have the same length in days.
Arguments
- x
Vector of datetimes.
- method
Method to calculate tetrad.
ebird(the default): The "ebird" method takes day 1-7 as tetrad 1, 8-14 as tetrad 2, 15-21 as tetrad 3 and >21 as tetrad 4. This method is employed by the Cornell Lab of Ornithology in their bar charts on the eBird platformfourths: the "fourths" method divides a month into four tetrads that are as close to equal in length as possible. A month with 30 days will consists of tetrads of length 7, 8, 7 and 8 in this order. A month with 28 days will consist of tetrads of length 7