[R] How to manipulate the time data without the date?

[tedzzx]

Hi,all

I only got the time data such as:
tms<-c("19:30:23","18:39:10".....)

I want to manipulate this time series data. For example, plus one second(or
minute) or minus one second

This data only has the time(h:m:s), without the date. I know that there are
chron package, ISOPix class and the timeDate class, but all these class need
the input of date. 

How can we manipulate the time data without the date?

Thanks advance

Ted
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