Alternative method for range-matching within 2 nested loops in R?

Hi.

I've been puzzling about how to replace the nested loops below. The idea is
that the B dataframe has rows with a posix datetime and the C dataframes has
posix Start and End times.  I want to assign a value to the observations in
B based in intersecting the appropriate time-interval in C.  I haven't been
able to discern a more efficient way to do this.  Any suggestions would be
most appreciated.

brows = dim(B)[1]
mrows = dim(C)[1]

for (i in 1:brows ) {
    for (j in 1:mrows ) {
        if (B$Datetime[i] >= C$DT_Start[j] & B$Datetime<=C$DT_End[j]){
            B$Site[i] = C$Proximity[j]
        }
    }
}

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John Helly, University of California, San Diego / San Diego Supercomputer
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