[R] histogram of time-stamp data

[e-letter]

Readers,

A data set consists of time-stamp values:

00:00:00
23:11:00
06:22:00

The data set was imported:

timestamps<-read.table("path/to/timestampsvalues")
hist(timestamps)

...error... x must be numeric

Then tried:

plot(timestamps).

How to adjust the graph to create a histogram where the intervals
ranges can be specified, e.g. intervals of 60 minutes?

Thanks in advance.