where Vdiode is when the diode's "on" current is of Vcc/R1 which can be determined from its datasheet or by testing. As an extreme example, when Vcc = 5 V and Vdiode= 0.7 V, high time = 1.00 R1C which is 45% longer than the "expected" 0.693 R1C. At the other extreme, when Vcc= 15 V and Vdiode= 0.3 V, the high time = 0.725 R1C which is closer to the expected 0.693 R1C. The equation reduces to the expected 0.693 R1C if Vdiode = 0 V.