Parsing a Time String with JavaScript

Categories: JavaScript

Tags: Algorithms

Let’s say you are building a UI where you’d like a user to enter a time into a text box.  This time value they might enter needs to handle a lot of different formats, but we’d like it to end up as the same value either way:

  • 1pm
  • 1:00pm
  • 1:00p
  • 13:00

Here’s a little JavaScript function to do this work.  Most of the interesting bits are the regular expression which helps handle of these various scenarios.  It takes a string representation of a time and attempts to parse it, setting the time on the specified date object sent to the function as the second parameter.  In the case where you do not provide a second parameter, the current date will be used.

function parseTime(timeStr, dt) {
    if (!dt) {
        dt = new Date();

    var time = timeStr.match(/(\d+)(?::(\d\d))?\s*(p?)/i);
    if (!time) {
        return NaN;
    var hours = parseInt(time[1], 10);
    if (hours == 12 && !time[3]) {
        hours = 0;
    else {
        hours += (hours < 12 && time[3]) ? 12 : 0;

    dt.setMinutes(parseInt(time[2], 10) || 0);
    dt.setSeconds(0, 0);
    return dt;

This function will return NaN if it can’t parse the input at all.  The logic immediately following the match() call is to handle the noon/midnight case correctly. 

Here’s a jsFiddle of this in action:

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