Shadow clocks (not sundials) predate "water clocks". The symbols of the months of the egyptian calendar are lunar based, thus prior to using a "civil" or solar calendar system (365 days) a lunar calendar system was used. The earliest reference to a calendric sequence occurs in the PT's and it generally accepted that a 365 day calendar was well in place by the time of the Old Kingdom because they're inscribed in tombs as "offering lists" to be performed on specific feasts throughout the "year" ;

Example:

Tomb of Ptahshepses (middle 5th dynasty) lists -

A boon that anubis, who is in front of the divine booth, who is lord of the Holy Land on his mountain and who is "the One in the Embalming Place (imy-wt), gives that there be [made] invocation-offerings of bread an beer for him (Ptahshepses) at the feast of the Opening of the Year (wp rnpt), the Feast of Thoth (dhwtyt) the Feast of the Head (possibly the first) of the Year (tp rnpt), The Feast of Wag, the Feast of Seker (hb skr), the Great Feast (hb wr), the Feast of the Burning (rkh), the Feast of Sadj, the Feat of the first day of the month, the Feast of the Half month, and every feast every day...

As far as the earliest attestation of Calendar use there are many many Old Kingdom invocation lists, supply lists, etc.... that show a calendar was in use at least as early as that period. If I am not mistaken though the earliest document (artifact) with a calendar arrangement is the well known Ebers Calendar (circa 1544-1537 BCE ~ Djeserkare, the throne-name of Amenhotep I).

Best Regards,

B.A. Hokom