subtract 1 minute from time Javascript [duplicate]

From my front end, somebody will submit the time using <input type="time">. So on the backend, it is retrieved like so:

let eventDetails = {
    eventStartTime: req.body.eventStartTime
}

so when I log this, it returns this (as a string), like so:

13:00

now what I need to happen is for it to subtract 1 minute from the time that gets returned, but I’m not sure where to start since it is technically a string. How can i do this?