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An hourglass (or sandglass, sand timer, or sand clock) is a device used to measure the passage of time.It comprises two glass bulbs connected vertically by a narrow neck that allows a regulated trickle of material (historically sand) from the upper bulb to the lower one.The earliest recorded reference that can be said with certainty to refer to a marine sandglass dates from c.
In the same period it appears in other records and lists of ships stores.
The hourglass, however, did not disappear entirely.
Although they became relatively less useful as clock technology advanced, hourglasses remained desirable in their design.
They can be given as a date/datetime object or text (in the ISO 8601 format).
If the start date is in the past, the trigger will not fire many times retroactively but instead calculates the next run time from the current time, based on the past start time.