Check if list of dates fall between a list of date ranges then add associated values to new dataframe using R programming

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R

I have a dataframe (df) with 4 columns: Start date, Stop date, Launch date, and NMBR (Usage amount - numerical value). I have another dataframe (usage) with two columns: time1 and usage. I am trying to check if each date in the time1 column in the usage dataframe falls between any of the start and end dates in the df dataframe. If it does fall in between the start and end date then capture the NMBR, store, and increment in the usage column in the usage dataframe. I am getting the error

"Error in as.POSIXlt.character(x, tz, ...) : character string is not in a standard unambiguous format".

I am new to r programming but would like help figuring out how to resolve.

What I have tried:

Here is sample code to help understand the logic I am going after:

time1 = seq(ISOdate(2018,09,17),ISOdate(2019,09,17), by = "hour")
usage = matrix(0,length(time1),1)

usage <- data.frame(time1,usage)

for (row in 1:nrow(df)) {
starttime <- df[row, "STRT"]
stoptime <- df[row, "STOP"]
launchtime <- df[row,"LNCHD"]
ncpu <- df[row,"NMBR"]

usage = apply(usage,1,add_ncpu)
add_ncpu = function(x){ if(x > starttime && x < stoptime){x = x + ncpu}}
}