aggregate function does not work: arguments must have same length

PLEASE READ THIS ENTIRE MESSAGE. It explains my problem and contains a reproducible example of the problem.

I am trying to run an aggregate function. The function produces the following error message:
ERROR MESSAGE: 
Error in aggregate.data.frame(as.data.frame(x), ...) : 
  arguments must have same length

Everything beyond this line can be pasted into an R session and run:

####Start of code
#I am trying to write a code that will compute the number of hours for which we have data for each day.
#Here are my data:

  inJan<-
    structure(list(NewTime = structure(c(1597363200, 1597366800, 
     1597370400, 1597374000, 1597377600, 1597381200, 1597384800, 1597388400, 
     1597392000, 1597395600, 1597399200, 1597402800, 1597406400, 1597410000, 
     1597413600, 1597417200, 1597420800, 1597424400, 1597428000, 1597431600, 
     1597435200, 1597438800, 1597442400, 1597446000, 1597449600, 1597453200, 
     1597456800, 1597460400, 1597464000, 1597467600, 1597471200, 1597474800, 
     1597478400, 1597482000, 1597485600, 1597489200, 1597492800, 1597496400, 
     1597500000, 1597503600, 1597507200, 1597510800, 1597514400, 1597518000, 
     1597521600, 1597525200, 1597528800, 1597532400, 1597536000, 1597539600
    ), class = c("POSIXct", "POSIXt"), tzone = "UTC"), Days = structure(c(18488, 
     18488, 18488, 18488, 18488, 18488, 18488, 18488, 18488, 18488, 
     18488, 18488, 18488, 18488, 18488, 18488, 18488, 18488, 18488, 
     18488, 18488, 18488, 18488, 18488, 18489, 18489, 18489, 18489, 
     18489, 18489, 18489, 18489, 18489, 18489, 18489, 18489, 18489, 
     18489, 18489, 18489, 18489, 18489, 18489, 18489, 18489, 18489, 
     18489, 18489, 18490, 18490), class = "Date")), class = "data.frame", row.names = c(NA,-50L))
inJan
names(inJan)

#I have written a function which finds the number of hours between a last time last(x) and a first time first(x).

# Number of follow-up hours in each day
timediff <- function(x) {
difftime(last(x),first(x),units="hours")
 }

#The function I wrote is used in an aggregate function which 
# finds the minimum and maximum time in each day, and then
# calls timediff to find the number of hours in each day.
zoop <- aggregate(inJan[,"NewTime"],as.list(inJan[,"Days"]),timediff)

#When I run the aggregate function I receive and error:
#Error in aggregate.data.frame(as.data.frame(x), ...) : 
#arguments must have same length

#HOWEVER I believe inJan[,"NewTime"] and inJan[,"Days"] have the same length   

llength(inJan[,"NewTime"]) 
#[1] 50
length(inJan[,"Days"])  
#[1] 50

#Can someone tell my why the aggregate function thinks the arguments are a different length?
#### End of code

Thank you,
John

John David Sorkin M.D., Ph.D.
Professor of Medicine, University of Maryland School of Medicine;
Associate Director for Biostatistics and Informatics, Baltimore VA Medical Center Geriatrics Research, Education, and Clinical Center; 
Former PI Biostatistics and Informatics Core, University of Maryland School of Medicine Claude D. Pepper Older Americans Independence Center;
Senior Statistician University of Maryland Center for Vascular Research;

Division of Gerontology, Geriatrics and Palliative Medicine,
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524
Cell phone 443-418-5382