To compute the timestamps for the first and last days of the current and previous months in PHP, developers commonly use built-in date and time functions such as mktime(), strtotime(), and date(). Below are practical and reliable approaches to achieve this.

Using mktime() for Month Boundaries

The mktime() function constructs a Unix timestamp from a date. For the current month:

$startOfCurrentMonth = mktime(0, 0, 0, date('n'), 1, date('Y'));
$endOfCurrentMonth   = mktime(23, 59, 59, date('n'), date('t'), date('Y'));

For the previosu month:

$startOfLastMonth = mktime(0, 0, 0, date('n') - 1, 1, date('Y'));
$endOfLastMonth   = mktime(23, 59, 59, date('n'), 0, date('Y'));

Note: Passing 0 as the day in mktime() automatically resolves to the last day of the previous month.

Alternative with strtotime()

A more readable approach uses relative date strings with strtotime():

$currentYear  = date('Y');
$currentMonth = date('m');

// Current month
$monthStart = strtotime("$currentYear-$currentMonth-01 00:00:00");
$monthEnd   = strtotime("$currentYear-$currentMonth-" . date('t') . " 23:59:59");

// Previous month
$lastMonthStart = strtotime('first day of last month 00:00:00');
$lastMonthEnd   = strtotime('last day of last month 23:59:59');

This method leverages PHP’s natural language date parser, which is less error-prone when handling month transitions (e.g., January to December of the prior year).

Formatting for Database Queries

Once timestamps are obtained, they can be formatted for SQL queries:

$startDate = date('Y-m-d', $startOfLastMonth);
$endDate   = date('Y-m-d', $endOfLastMonth);
$whereClause = "BETWEEN '$startDate' AND '$endDate'";

This ensures clean integration with database systems that expect date strings in YYYY-MM-DD format.