The output of django.utils.timesince() will become increasingly inaccurate as the number of leap years in the supplied time period grows.
refs #19210 Added test to verifiy the behavior while long time
refs #19210 Changed seconds of year in chanks
refs #19210 Fixed not to use dateutil
refs #19210 Solved (with dirty implementation)
refs #19210 Fixed behivior on continuance into the new year
refs #19210 Added test to check multiple units
Merge https://github.com/django/django into t19210
Please open a new PR if you can update this to merge cleanly to master, thanks!