This can not always be done and when it can, it should be done with care. A 90deg lead might as well be a 90deg lag to the anti-phase. There is therefore a non-uniqueness problem when doing the conversion. A phase angle can also only be converted to a time lag for a specific wavelength. This equation works best for determining the time lag when the series are near in-phase.
wavelength=11; phaseangle=20*pi/180; timelag=phaseangle*wavelength/(2*pi)A visual inspection of the time series at the wavelength in question should make it clear if the time lag is right. I also recommend calculating the time lag with other methods for support.