Time at the Abode flows differently from time in the "wide world."
Marking time in the wide world is, of course, done in minutes, hours, months, years...
Within the mists, however, none of those elements apply. The children know only that they wake up and begin work during "the lightening" and sleep during "the darkening." Lightenings and darkenings can be counted, as when the character Molly tells her friend Fel how many darkenings have passed since Fel's escape. Fel, who upon her return has been away from the Abode for two years, is astonished that only 14 lightenings and darkenings have passed in Molly's time.
However, since each lightening and darkening is the same as every other, there's never much reason for the children to count them. The Smilts, the couple who control everything at the Abode, may count lightenings and darkenings between appearances of The Visitor, but they're not telling.
As the plot of The Abode moves forward, the reason for the time differences reveals itself. There's human time and then there's...well, I'm not telling, either. More than one secret lies within the mists.