[R] Need help with labeling a plot correctly

[Neotropical bat risk assessments]

Hi all,
Eons ago (~2007) Hadley Wickham extremely generously helped me developed 
a means for plotting temporal activity w/ ggplot.
I need to revisit this and have it working again.  I failed to copy the 
entire legacy code over to my traveling laptop from my workstation, but 
found a partial bit of the code in an old 2009 Gmail attachment.  I am 
slated to do a graduate seminar at UF G'ville and needed to discuss 
temporal activity.

I have been working my feeble brain again and revisiting the almost 
complete code and have it partially working by doing step by step 
sections of code and carefully making sure no typos that could screw up 
syntax.

The partial code Hadley developed for me is below and is now working 
more or less with R 3.4.3 (2017-11-30).
What I need to do is have the missing bit of code so that the Y- Axis of 
the plots has time of night  rather than the "elapsed seconds."
As a note this is for bat activity data so data points begin after 
sunset on a given night and end at sunrise the following day so 
date/times span 2 calender days with a midnight roll-over.

The code below was provided by Hadley and works except for the Y axis 
labeling with correct times.
I can email a sample of the raw data if anyone is able to help.

Cheers,

Bruce
++++++++++++++++++++++

library(zoo)
library(cluster)
library(lattice)
library(vegan)
library(chron)
library(MASS)
library(ggplot2)

# activity plots that Hadley Wickham helped develop back in ~2007
#New from Hadley
*s*etwd("C:/Bats/Temporal data")

#C:\Bats\Temporal data
source("date.r")
All <- read.csv("C:/Bats/Temporal data/TECTemporal.CSV")

  Ptepar <-subset(All, Species == "Ptepar")
str(Ptepar)

# Notice that date and time are factors - you need to convert them
# into dates.  This doesn't happen automatically in R.  Also, the syntax to
# make dates isn't exactly intuitive.

Ptepar$Date <- as.Date(Ptepar$Date, "%m/%d/%Y")

Ptepar$Time <- as.POSIXct(strptime(as.character(Ptepar$Time), "%H:%M"))
Ptepar$datetime <- as.POSIXct(strptime(paste(Ptepar$Date, 
format(Ptepar$Time,
"%H:%M")), "%Y-%m-%d %H:%M"))

# For a given time, give the time the evening started
# (this will be the previous day if the time is in the morning)
evening <- function(date) {
  update(date,
    yday = yday(date) - ifelse(am(date), 1, 0),
    hour = 18,
    minute = 0,
    second = 0)
}

# Now we calculate the number of seconds since the start of the evening
Ptepar$elapsed <- as.double(Ptepar$datetime - evening(Ptepar$datetime), 
"secs")
ggplot(Ptepar, aes(Date, elapsed, colour = Location)) +
  geom_jitter() +
  facet_wrap(~ Species)

The resulting plot appears fine however I need to have the Y axis 
display time of night rather than the elapsed seconds.